24.2 bacteria
The domains archaea and eubacteria contain microorganisms that are commonly referred to as bacteria. Another common name for them is germs. Some unusual bacteria have the genetic ability to in extreme environments such as sulfur hot springs, on glaciers, and at the openings of submarine volcanic vents. They are single-celled prokaryotes that lack an organized nucleus and other complex organelles. Bergey’s Manual of Determinative Bacteriology first published in 1923 now lists in its latest edition over 2000 species of bacteria and describes the subtle difference among them. As investigators have discovered more bacteria, they have come to suspect that the known species may represent only 1% of all the bacteria on earth. For general purposes, bacteria are divided into the three groups based on such features as their staining properties, ability to form endospores, shape, motility, bolism, and reproduction. Table 24.1 shows the most generally accepted taxonomy of the bacteria.
Many forms of bacteria are beneficial to humans. Some forms of bacteria decompose dead material, sewage, and other wastes into simpler molecules that can be recycled. Organisms that in this manner are called saprophytic. The food industry uses bacteria to produce cheese, yogurt, sauerkraut, and many other foods. Alcohols, acetones, acids, and other chemicals are produced by bacterial cultures. The pharmaceutical industry employs bacteria to produce antibiotics and vitamins. Some bacteria can even bolize oil and are used to clean up oil spills.
There are also mutualistic relationships between bacteria and other organisms. Some intestinal bacteria benefit humans by producing antibiotics that inhibit the development of pathogenic bacteria. They also compete with disease-causing bacteria for nutrients, thereby helping keep the pathogens in check. They aid digestion by releasing various nutrients. They produce and release vitamin K.  mutualistic bacteria establish this symbiotic relationship when they are ingested along with food or drink. When people travel, they consume local bacteria along with their food and drink and may have problems establishing a new symbiotic relationship with these foreign bacteria. Both the host and the symbionts have to make adjustments to their new environment, which can result in a very uncomfortable situation for both. Some people develop traveler’s diarrhea as a result.
Animals do not produce the enzymes needed for the digestion of cellulose. Methanogens, bacteria that obtain bolic energy by reducing carbon dioxide to methane, digest the cellulose consumed by herbivorous animals, such as cows, thereby permitting the cow to obtain simple sugars from the otherwise useless cellulose. There is a mutualistic relationship between the cow and methanogens. Some methanogens are also found in the human gut and are among the organisms responsible for the production of intestinal gas. In some regions of the world methanogens are used to digest organic waste, and the methane is used as a source of fuel.
The romans knew that bean plants somehow enriced the soil, but it was not until the 1800s that bacteria were recognized as the enriching agents. Certain types of bacteria have a symbiotic relationship with the roots of bean plants and other legumes. These bacteria are capable of converting atmospheric nitrogen into a form that is usable to the plants.
Early forms of life consisted of prokaryotic cells living in a reducing atmosphere. Photosynthetic bacteria released oxygen, and earth’s atmosphere began to change to an oxidizing atmosphere. Photosynthetic, colonial blue-green bacteria are still present in large numbers on earth and continue to release significant quantities of oxygen. Colonies of blue-green bacteria are found in aquatic environments, where they form long, filamentous strands commonly called pond scum. Some of the large cells in the colony are capable of nitrogen fixation and convert atmospheric nitrogen, N2, to ammonia, NH3. This provides a form of nitrogen usable to other cells in the colony—an example of division of labor.
The word bacteria usually brings to mind visions of tiny things that cause disease; however, the majority are free living and not harmful. Their roles in the ecosystem include those of decomposers, nitrogen fixers, and other symbionts. It is true that some diseases are caused by bacteria, but only a minority of bacteria are pathogens, microbes that cause infectious diseases. It is normal for all organisms to have symbiotic relationships with bacteria. Most organisms are lined and covered by populations of bacteria called normal flora. In fact, if an organism lacks bacteria it is considered abnormal. Some pathogenic bacteria may be associated with an organism yet do not cause disease. For example, streptococcus pneumoniae may grow in the throats of healthy people without any pathogenic effects. But if a person’s resistance is lowered, as after a bout with viral flu, streptococcus pneumoniae may reproduce rapidly in the lungs and cause pneumonia; the relationship has changed from commensalistic to parasitic.
Bacteria may invade the healthy tissue of host and cause disease by altering the tissue’s normal physiology. Bacteria living in the host release a variety of enzymes that cause the destruction of tissue. The disease ends when the pathogens are killed by the body’s defenses or some outside agent, such as an antibiotic. Examples are the infectious disease strep throat, syphilis, pneumonia, buberculosis, and leprosy.
Many other bacteria illnesses are caused by toxins or poisons produced by bacteria that may be consumed with food or drink. In this case, disease can be caused even through the pathogens may never enter the host. For example, botulism is an extremely deadly disease caused by the presence of bacteria toxins in food or drink. Some other bacteria disease are the result of toxins released from the bacteria growing inside the host tissue; tetanus and diphtheria are examples. In general, toxins may cause tissue damage, fever, and aches and pains.
Bacteria pathogens are also important factors in certain plant disease. Bacteria are the causative agents in many types of plant blights, wilts, and soft rots. Apples and other fruits trees are susceptible to fire blight, a disease that lowers the fruit yield bacause it kills the tree’s branches. Citrus canker, a disease of citrus fruits that causes cancerlike growths, can generate widespread damage. In a three-year period, florida citrus grower lost $2.5 billion because of this disease.
Despite large investments of time and money, scientists have found it difficult to control bacteria populations. Two factors operate in favor of the bacteria: their reproductive rate and their ability to form spores. Under ideal conditions some bacterial can grow and divide every 20 minutes. If one bacterial cell and all its offsprings were to reproduce at this ideal rate, in 48 hours there would be 2.2×1043cells. In reality, bacteria cannot achieve such incredibly large populations because they would eventually run out of food and be unable to dispose of their wastes.
Because bacteria reproduce so rapidly, a few antibiotic resistant cells in a population can increase to dangerous levels in a very short time. This requires the use of stronger doses of antibiotics or new types of antibiotics to bring the bacteria under control. Furtherfore, these resistant strains can be transferred from one host to another. For example, sulfa drugs and penicillin, once widely used to fight infections, are now ineffective against many strains of pathogenic bacteria. As new antibiotics are developed, natural selection encourages the development of resistant bacteria strains. Therefore humans are constantly waging battles against new strains of resistant bacteria.
Another factor that enables some bacteria to survive a hostile environment is their ability to form endospores. An endospore is a unique bacteria structure with a low bolic rate that can geminate under favorable conditions to form a new, actively growing cell. For example, people who preserve food by canning often boil the food in the canning jars to kill the bacteria. But not all bacteria are killed by boiling; some of them form endospores. For example, botulism poison is usually found in foods that are improperly canned. The endospores of Clostridium botulinum, the bacterium that causes botulism, can withstand boiling and remain for years in the endospore state. However, endospores do not germinate and produce botulism toxin if the PH of the canned goods is in the acid range; in that case, the food remains preserved and edible. If conditions become favorable for endospores to germinate, they become actively growing cells and produce toxin. Home canning is the major source of botulism. Using a pressure cooker and heating the food to temperatures higher than 121 oC for 15 to 20 minutes destroys both botulism toxin and the endospores.

24.3 Kingdom protista
The first protists evolved about 1.5 billion years ago. Like the prokaryotes, most of the protists are one-celled organisms. However, there is a significant difference between the two kingdoms: all the protists are unkaryotic cells and all the prokaryotes are prokaryotic cells. Prokaryotic cells usually hav a volume of 1 to 5 cubic micrometers. Most eukaryotic cells have a volume greater than 5000 cubic micrometers. This means that eukaryotic cells usually hav a volume at least 1000 times greatert than prokaryotic cells. The presence of membranous organelles such as the nucleus, endoplasmic reticulum, mitochondria, and chloroplasts allows protists to be larger than prokaryotes. These organelles provide a much greater surface area within the cell upon which specialized reactions may occur. This allows for mor efficient cell bolism than is found in prokaryotic cells.
Because of the great diversity within the more than 60,000 species, it is a constant challenge to separate the kingdom protista into subgroupings as research reveals new evidence about members of this group. Usually the species are divided into three groups: algae, autotrophic unicellular organisms: protozoa, heterotrophic unicellular organisms; and funguslike protists. However, emergeing evidence suggests a much more complex evolutionary pattern as noted in the cladogram seen in the table 24.3.

Plantlike protists
Algae are protists that have a cellulose cell wall. They contain chlorophyll and can therefore carry on photosysthesis. Unicellular and colonial types occur in a variety of habitats. There are two major forms of algae in a variety of marine and freshwater habitats: planktonic and benthic. Plankton consists of small floating or weakly swimming organisms. Benthic organisms live attached to the bottom or to objects in the water. Phytoplankton consists of photosynthetic planton that forms the basis for most aquatic food chains. The large number of benthic and planktonic algae makes them an important source of atmosphere oxygen. It is estimated that 30% to 50% of atmosphere oxygen is produced by algae.
Because algae require light, phytoplankton is found only near the surface of the water. Even in the clearest water, phtosysthesis does not usually occur any deeper than 100 meters. To remain near the surface, some of the phytoplankton are capable of locomotion. Others maintain their position by storing food as oil, which is less dense than water and enables the cells to float near the surface.
Three common forms of single-celled algae typically found as phytoplankton are the euglenophyta and chrysophyta, and pyrrophyta (dinoflagellates). Euglena are found mainly in freshwater. They are widely studied because they are easy to culture. Under low levels of light, these photosynthetic species can ingest food. Euglena can be either autotrophic or heterotrophic.
There are over 10,000 species of diatoms. Diatoms are commonly found in freshwater, marine and soil environments. They can reproduce both sexually and asexually. When conditions are favorable, asexual reproduction can result in what is called an algae bloom—a rapid increase in the population of microorganisms in a body of water. The population can become so large that the water looks murky. These algae are unique because their cell walls contain silicon dioxide. The algal walls fit together like the lid and bottom of a shoe box; the lid overlaps the bottom. Because their cell walls contain silicon dioxide, they readily form fossils. The fossil cell walls have large, abrasive surface area with many tiny holes and can be used in a number of commercial processes. They are used as filters for liquids and as abrasives in specialty soaps, toothpastes, and scouring powders.
Along with the diatoms, dinoflagellates are the most important food producers in the ocean’s ecosystem. All members of this group of algae have two flagella, which is the reason for their name. many marine forms are bioluminescent; they are responsible for the twinkling lights at night in ocean waves or in a boat’s wake.
Some species of dinoflagellates have symbiotic relationships with marine animals, such as the reef corals; the dinoflagellats provide a source of nutrients for the reef-building coral. Corals that live in the light and contain dinoflagellates grow 10 times faster than corals without this symbiont. Thus, in coral reef ecosystems, dinoflagellates form the foundation of the food chain. Some forms of dinoflagellates produce toxins that can be accumulated by such filter-feeding marine animals as clams and oysters. Filter-feeding shellfish ingest large amounts of the toxins, which has no effect on the shellfish but can cause sickness or death in animals that feed on them, such as fish, birds, and mammals. Many of the toxin-producing dinoflagellates contain red pigment. Blooms of this kind are responsible for red tides. Red tides usually occur in the warm months, during which people should refrain from collecting and eating oysters. The expression “oysters ‘R’ in season” comes from the fact that most of the months with an R in their spelling are cold weather months, during which oysters are safer to eat. Commercially available are tested for toxin content; it they are toxic, they are not marketed. Red tides not only have occurred off the coast of Florida in North America, but also have more recently developed off the coast of china. Hundreds of thousands of fish and other marine life have been killed as a result of toxin release, thus having a significant impact on the economy and food supply.
In recent years a new problem has surfaced caused by the dinoflagellate, Pfiesteria piscidia. These algae have been responsible for the death of millions of fish in estuaries of the eastern United States. These dinoflagellates release toxins that paralyze fish and feed on the fish. They have also been responsible for human and wildlife poisoning.
Multicellular algae, commonly known as seaweed, are large colonial forms usually found attached to objects in shallow water. Two types, red algae and brown algae, are mainly marine forms. The green algae are a third kind of seaweed; they are primarily freshwater species.
Red algae live in warm oceans and attach to the ocean floor by means of a hold faster structure. The may be found from the splash zone, the area where waves are breaking, to depths of 100 meters. Some red algae become encrusted with calcium carbonate and are important in reef building; other species are of commercial importance because they produce agar and carrageenin. Agar is widely used as a jelling agent for growth media in microbiology. Carrageenin is a gelatinous material used in paints, cosmetics, and baking. It is also used to make gelatin desserts harden faster and to make ice cream smoother. In asia and Europe some red algae are harvested and used for food.
Brown algae are found in cooler marine environments than are the red algae. Most species of brown algae have a hold fast organ. Colonies of these algae can reach 100 meters in length. Brown algae produce alginates, which are widely used as stabilizers in frozen desserts, emulsifiers in salad dressings, and as thickeners that give body to foods such as chocolate milk and cream cheeses; they are also used to form gels in such products as fruit jellies.
The Sargasso sea is a large mat of free-floating brown algae between the Bahamas and the Azores. It is thought that this huge mass is the result of brown algae that have become detached from the ocean bottom, have been carried by ocean currents, and accumulate in this calm region of the Atlantic Ocean. This large mass of floating algae provides a habitat for a large number of marine animals, such as marine turtles, eels, jellyfish, and innumerable crustaceans.
Green algae are found primarily in fresh water ecosystems, where they may attach to a variety of objects. Members of this group can also be found growing on trees, in the soil, and even on snowfields in the mountains.

23.2 domains archaea and eubacteria
Member of the domain archaea and eubacteria are commonly known as bacteria. Some are disease-causing, such as Streptococcus pneumoniae, but most are not. In addition, many are able to photosynthesize. Members of these domains differ from one another in their cellular structures and position on the evolutionary tree. Evidence gained from studying DNA and RNA nucleotide sequences and a comparison of the amino acid sequences of proteins indicates that the first ancestral cells used DNA as their genetic material. They probably gave rise to today’s eubacteria followed by the arhaea and finally the eucarya.
Archaea
The term archaea comes from the term archaios meaning “ancient”. This group of protaryotic cells is thought to have branched off in the neighborhood of 1.3 to 2.6 million years ago. Since the enbacteria are actually older as a group than the archaea, the naming of this group may seem confusing. However, the archaea have many chemical similarities to the eucarya. For example, the archaea and eucarya both lack peptioglycan as their cell wall building material and they both have introns as components of their DNA. These bacteria have been found in many shapes including rods, spheres, spirals, filaments, and flat plates. Because they are found in many kinds of extreme environments, they have become known as extremophiles. Based on this fact, the archaea are divided into three groups. Mathanogens are methane producing bacteria that are anaerobic. They can be bound in the intestinal tracts of humans, sewage, and swamps. Halobacteria are found growing in very salty environments such as the Great Salt Lake, salt ponds, and brine solutions. Some contain a special kind of chlorophyll and are therefore capable of generating their ATP by photosynthesis. The thermophilic archaea live in environments that normally have very high temperatures and high concentration of sulfur. Over 500 species of thermophiles have been identified at the openings of hydrothermal vents in the open oceans. One such thermophile, pyrolobus fumarii, grows in a hot spring in Yellowstone National Park. Its maximum growth temperature is 113 oC, its optimum is 106 oC, and its minimum is 90 oC.
Eubacteria
The true bacteria are small, single-celled organisms ranging from 1 to 10 micrometers. Their cell walls typically contain complex organic molecules, such as peptidoglycan, polymers of unique sugars, and unusual amino acids not found in other kinds of organisms.
   Eubacteria have no nucleus, and the genetic material is a single loop of DNA. Some have as few as 5000 genes. The cells reproduce by binary fission. This is a type of asexual cell division that does not involve the more complex structures used by eukaryotes n mitosis or meiosis. As a result, the daughter cells produced have a single copy of the parental DNA loop. Some cells move by secreting a slime that glides over the cell’s surface, causing it to move through the environment. Others move by means of a kind of flagellum. The structure of the flagellum is different from the flagellum found in eukaryotic organisms.
Because the early atmosphere is thought to have been a reducing atmosphere, the first enbacteria were probably anaerobic organisms. Today there are both anaerobic and aerobic eubacteria.
Some prokaryotic heterotrophs are saprophytes, organisms that obtain energy by the decomposition of dead organic material; others are parasites that obtain energy and nutrients from living hosts and cause disease; still others are mutualistic and have a mutually beneficial relationship with their host; finally,some are commensalistic and derive benefit from a host without harming it. Several kinds of eubacteria are autotrophic. Many are called cyanobacteria because they contain a blue-green pigment, which allows then to capture sunlight and carry on photosynthesis. They can become extremely numerous in some polluted waters where nutrients are abundant. Others use inorganic chemical reactions for their energy sources and are called chemosynthetic.
Some biologist hypothesize that the eukaryotic cells evolved from prokaryotic cells by a process of endosymbiosis. This hypothesis proposes that structures like mitochondria, chloroplasts, and other membranous organelles originated from separate cells that were ingested by large, more primitive cells. Once inside, these structures and their s became integrated with the host cell and ultimately became essential to its survival. This new type of cell was the forerunner of present-day eukaryotic cells. Single-celled eukaryotic organisms are members of the kingdom protista.
23.3 domain eucarya
Kingdom protista
The changes in cell structure that led to eukaryotic organisms most probably gave rise to single-celled organism similar to those currently grouped in the kingdom protista. Most members of this kingdom are one-celled organisms, although there are some colonial forms. Eukaryotic cells are usually much larger than the prokaryotes, typically have more than a thousand times the volume of prokaryotic cells. Their larger size was made possible by the presence of specialized membraneous organisms, such as mitochondria, the endoplasmic reticulum, chloroplasts and nuclei.
There is a great deal of diversity within the 60,000 known species of protista. Many species live in freshwater; others are found in marine or terrestrial habitats, and some are parasitic, commenslistic, or mutualistic. All species can undergo mitosis, resulting in asexual reproduction. Some species can also undergo meiosis and reproduce sexually. Many contain chlorophyll in chloroplasts and are autotrophic; others require organic molecules as sources of energy and are heherotrophic. Both autotrophs and heterotrophs have mitochondria and respire anaerobically.
Because members of this kingdom are so diverse with respect to form, bolism, and reproductive methods, most biologists do not think that protista form a valid phylogenetic unit. However, it is still a convenient taxonomic grouping. By placing these organisms together in this group it is possible to gain a useful perspective on how they relate to other kinds of organisms. After the origin of eukaryotic organisms, evolution proceeded along several different pathways. Three major lines of evolutions can be seen today in the plantlike autotrophs, animal-like heterotrophs, and the funguslike heterotrophs. Amoeba and paramecium are commonly encountered examples of protozoa. Many seaweeds and pond scums are collections of large numbers of algal cells. Slime molds are less frequently seen because they live in and on the soil in moist habitats; they are most often encountered as slimy masses on decaying logs.
Through the process of evolution, the plantlike autotrophs probably gave rise to the kingdom plantae, the animal-like heterotrophs probably gave rise to the kingdom animalia, and the funguslike hetertrophs were probably the forerunners of the kingdom fungi.

Kingdom fungi
Fungus is the common name for members of the kingdom fungi. The majority of fungi are nonmotile. They have a rigid, thin cell wall, which in most species is composed of chitin, a complex carbonhydrate containing nitrogen. Members of the kingdom fungi are nonphotosynthetic, eukaryotic organisms. The majority (mushrooms and molds) are multicellular, but a few like yeasts, are single-celled. In the multicellular fungi the basic structural unit is a network of multicellular filaments. because all of these organisms are heterotrophs, they must obtain nutrients form organic sources. Most are saprophytes and secret enzymes that digest large molecules into smaller units that are absorbed. They are very important as decomposers in all ecosystems. They feed on a variety of nutrients ranging from dead organisms to such products as shoes, food-stuffs,and clothing. Most synthetic organic molecules are not attached as readily by fungi; this is why plastic bags, foam cups, and organic pesticides are slow to decompose.
Some fungi are parasitic, whereas others are mutualistic. Many of the parasitic fungi are important plant pests. Some attack and kill plants (chestnut blight, dutch elm disease); others injure the fruit, leaves, roots, or stems and reduce yields. The fungi that are human parasites are responsible for athlete’s foot, vaginal yeast infections, valley fever, “ringworm”, and other disease. Mutualistic fungi are important in lichens and in combination with the roots of certain kinds of plants.
Kingdom plantae
Another major group with roots in the kingdom protista are green, photosynthetic plants. The ancestors of plants were most likely specific kinds of algae commonly called green algae. Members of the kingdom plantae are nonmotile, terrestrial, multicellular organisms that contain chlorophyll and produce their own organic compounds. All plant cell have a cellulose cell wall. Over 300,000 species of plants have been classified; about 85% are flowering plants, 14% are mosses and ferns, and the remaining 1% are cone-bearers and several other small groups within the kingdom.
   A wide variety of plants exist on Earth today, members of the kingdom plantae range from simple mosses to vascular plants with stems, roots, leaves, and flowers. Most biologists believe that the evolution of this kingdom began about 400 million years ago when the green algae of the kingdom protista gave rise to two lines: the nonvascular plants like the mosses evolved as one type of plant and the vascular plants like the ferns evolved as a second type. Some of the vascular plants evolved into seed-producing plants, which today are the cone-bearing and flowering plants, whereas the ferns lack seeds. The development of vascular plants was a major step in the evolution of plants from an aquatic to a terrestrial environment.
Plants have a unique life cycle. There is a haploid gametophyte stage that produces a haploid sex cell by mitosis, there is also a diploid sporophyte stage that produces haploid spores by meiosis. This alternation of generations, which is a unifying theme that ties together all members of this kingdom, is fully explained in chapter 25. in addition to sexual reproduction, plants are able to reproduce asexually.

Kingdom animalia
Like the fungi and plants, the animals are thought to have evolved from the protista. Over a million species of animals have been classified. These range from microscopic types, like mites or aquatic larvaes of marine animals, to huge animal like elephants or whales. Regardless of their types, all animals have some common traits. All are composed of eukaryotic cells and all species are heterotrophic and multicellular. All animals are motile, at least during some portion  of their lives; some like the sponges, barnacles, mussels, and corals, are sessile when they are most easily recognized—the adult portion of their lives. All animals are capable of sexual reproduction, but many of the les complex animals are also able to reproduce asexually.
It is thought that animals originated from certain kinds of protista that had flagella. This idea proposes that colonies of flagellated protista gave rise to simple multicellular forms of animals like the ancestors of present-day sponges. These first animals lacked specialized tissues and organs. As cells became more specialized, organisms developed special organs and systems of organs and the variety of kinds of animals increased.
Although taxonomists have grouped organisms into six kingdoms, some organisms do not easily fit into these categories. Viruses, which lack all cellular structures, still show some characteristics of life, in fact, some scientist consider them to be highly specialized parasites that have lost their complexsity as they developed as parasites. Others consider them to be the simplest of living organisms. Some even consider them to be nonliving. For these reasons viruses are considered separated from the six kingdoms.

23.4 acellular infectious particles
All of the group discussed so far fall under the category of cellular forms of life. They all have at least the following features in common. They have cell membranes, nucleic acids as their genetic material, cytoplasm, enzymes and coenzymes, ribosomes, and use ATP as their source of chemical-bond energy. Since the three groups to follow lack this cellular organization, they are referred as acellular or are known as infectious particles. In order for these to make more of their own kind, they must make their way into true cells where they become parasites eventually causing harm or death to their host cells. The only infectios particles that are considered beneficial are a new that have been modified through bioengineering to help in the genetic transformation of cells. One example of an infectious agent that has been “domesticated” is HIV, human immunodeficiency virus. Many bioengineers use this “tame” form of the virus to carry laboratory-attached genes into host animal cells in an attempt to change their genetic makeup. Since evolutionary biologists can only speculate on the origin of acellular infectious particles, they are not classified using the same methods outlines above. Therefore, the names of viruses, viroids, and prions are varied and may not seem logical.

Viruses
A virus consists of a nucleic acid core surrounded by a coat of protein. Viruses are obligate intrcellular parasites, which means they are infectious particles that can only when inside a living cell. Because of their unusual characteristics, viruses are not members of any of the three domains. Biologists do not consider them to be living because they are not capable of living and reproducing by themselves and show the characteristics of life only when inside living cells.
   Soon after viruses were discovered in the late part of the nineteenth century, biologists began to speculate on how they originated. One early hypothesis was that they were either prebionts of parts of prebionts that did not evolve into cells. This idea was discared as biologists learned more about the complex relationship between viruses and host cells. A second hypothesis was that viruses developed from intracellular parasites that became so specialized that they need only the nucleic acid to continue their existence. Once inside a cell, this nucleic acid can take over and direct the host cell to provide for all of the virus’s needs. A third hypothesis is that viruses are runaway genes that have escaped from cells and must return to a host cell to replicate. Regardless of how the viruses came into being, today they are important as parasites in all forms of life.
Viruses are typically host-specific, which means that they usually attack only one kind of cell. The host is a specific kind of cell that provides what the virus needs to . Viruses can infect only those cells that have the proper receptor sites to which the virus can attach. This site is usually a glycoprotein molecule on the surface of the cell membrane. For example, the virus responsible for measles attach to liver cells, and mumps viruses attach to cells in the salivary glands. Host cells for the HIV virus include some types of human brain cells and several types belonging to the immune system.
Once it has attached to the host cell, the virus either enters the cell intact or it injects its nucleic acid into the cell. If it enters the cell, the virus loses its protein coat, releasing the nucleic acid. Once released into the cell, the nucleic acid of the virus may remain free in the cytoplasm or it may link with the host’s genetic material. Some viruses contain as few as 3 genes, others contain as many as 500. a typical eukaryotic cell contains tens of thousands of genes. Most viruses need only a small number of genes because they rely on the host to perform most of the activities necessary for viral multiplication. Viruses do not “reproduce” as do true cells—that is, mitosis or meiosis. In those processes, the contants of the cell are doubled prior to splitting the cell into daughter cells. If automobiles “reproduced”, you would found your car parts doubling as time went by. Then one day you would discover that your “adult” car had reproduced by manufacturers; they are “replicated” as are viruses. Virus particles are recreated using a set of instruction and new building materials.
Some viruses have DNA as their genetic material but many have RNA. The RNA must first be reverse transcribed to DNA before the virus can reproduce. Reverse tranase, the enzyme that accomplishes this has become very important in the new field of molecular genetics because its use allows scientists to make large numbers of copies of a specific molecule of DNA.
Viral genes are able to take command of the host’s bolic pathways and direct it to carry out the work of making new copies of the original virus. The virus makes use of the host’s available enzymes and ATP for this purpose. When enough new viral nucleic acid and protein coat are produced, complete virus particles are assembled and released from the host. The number of viruses released ranges from 10 to thousands. The virus that causes polio releases about 10,000 new virus particles from each human host cell. Some viruses remain in cells and are only occasionally triggered to reproduce, causing symptoms of disease. Herps viruses, which cause cold sores, genital herpes, and shingles reside in nerve cells.
Viruses vary in size and shape, which helps in classifying them. Some are rod-shaped, others are round, and still others are in the shape of a coil or helix. Viruses are some of the smallest infecting agents known to humans. Only a few can be seen with a standard laboratory microscope; most require an electron microscope to make them visible. A great deal of work is necessary to isolate viruses from the environment and prepare them for observation with an electron microscope. For this reason, most viruses are more quickly identified by their activities in host cells. Almost all the species n the six kingdoms serve as hosts to some form of virus.

Viroids: infectious RNA
The term viroid refers to infectious particles that are only like viruses. Viroids are composed solely of a small, single strand of RNA. To date to no viroids have been found to parasitize animals. The hosts in which they have been found are cultivated crop plants such as potatoes, tomatoes, and cucumbers. Viroid infections result in stunted or distorted growth and may sometimes cause the plants to die. Pollen, seeds, or farm machinery can transmit viroids from one plant to another. Some scientists believe that viroids may be parts of normal RNA that have gone wrong.

Prions: infectious proteins
Several kinds of brain disease appear to be caused by proteins that can be passed from one individual to another. These infectious agents are called prions. All the diseases of this type currently known cause changes in the brain that result in a sponge appearance to the brain called spongiform encephalopathies. The symptoms typically involve abnormal behaviour and eventually death. In animals the most common examples are scrapie in sheep and goats and mad cow disease in cattle. Scrapie got its name because one of the symptoms of the disease is an itching of the skin associated with nerve damage that causes the animals to rub against objects and scrape their hair off.
The occurrence of mad cow disease in Great Britain was apparently caused by the spread of prions from sheep to cattle. This occurred because the practice of processing unusable parts of sheep carcasses into a protein supplement that was fed to cattle. Other similar diseases are known from mink, cats, dogs, elk, and deer. It now appears that the original form of BSE has changed to a variety that is able to infect humans. This new form is called vCJD which makes scientists believe that BSE and vCJD are in fact the same prion.
In humans there are several similar diseases. Kuru is a disease known to have occurred in the Fore people of the highlands of Papua New Guinea. The disease was apparently spread because the people ate small amounts of brain tissue of dead relatives. (This ritual is performed as an act of love and respect for the relative.) When the Fore people were encouraged to discontinue this ritual, the incidence of the disease declined. CJD is found throughout the world. Its spread is associated with medical treatment, i.e., tissue transplants. Contaminated surgical instruments and tissue transplants such as corneal transplants are the most likely cause of transfer from affect to uninfected persons.
It now appears to be well-established that these proteins can be spread from one animal to another and they do cause disease, but how are they formed and how do they multiply? The multiplication of the prion appears to result from the disease-causing prion protein coming in contact with a normal body protein and converting it into the disease-causing form, a process called conversion. Since this normal protein is produced as a result of translating a DNA message, scientists looked for the genes that make the protein and have found it in a wide variety of mammals. The normal allele produced a protein that does not cause disease, but is able to be changed by the invading prion protein into the prion form. Prions do not “reproduce” or replicate” as do viruses or viroids. A prionnecious protein presses up against a normal body protein and may cause it to change shape to that of the dangerous protein. When this conversion happens to a number of proteins, they stack up and interlock, as do the individual pieces of a Lego toy. When enough link together they have a damaging effect—they form plaques of protein on the surface of nerve cells that disrupt the flow of the nerve impulses and eventually cause nerve cell death. Brain tissues taken from animals that have died of such disease appear to be full of holes, thus the name spongiform encephalitis. Because infected organisms lose muscle mass and weight as a result of prion infection, these disease are now called chronic wasting diseases. A person’s susceptibility to acquiring a prion disease such as CJD depends on many factors, among them their genetic makeup. If the normal protein is of a significantly different amino acid sequence, the prion may not be able to convert it to its own dangerous form. These abnormal proteins are resistant to being destroyed by enzymes and most other agents used to control infectious diseases. Therefore individuals with the disease-causing form of protein can serve as the source of the infectious prions.
There is still much to learn about the of the prion protein and how the abnormal, infectious protein can cause copies of itself to be made. A better understanding of the alleles and the proteins they make will eventually lead to effective treatment and prevention of these serious disease in humans and other animals.

22.2 Current thinking about the origin of life
Although Pasteur thought that he had defeated those that believed in spontaneous generation and strongly supported biogenesis, we still have modifications of these two major scientific theories regarding the origin of life today. One holds that life arrived on earth from some extraterrestrial source (biogenesis) and the other maintains that life was created on Earth from nonliving material (spontaneous generation). Early in the 1900s, Svante Arrhenius proposed a different twist on biogenesis. His concept, called panspermia, hypothesized that life arose outside the earth and that living things were transported to earth serving to seed the planet with life. While his ideas had little scientific support at that time, his basic idea has since been revived and modified as result of new evidence gained from space explorations, as you will see later in the chapter. However, panspermia does not explain how life arose originally. Explanations of how life might have originated are now focused on chemical theories. The chemical theories suggest that life arose from natural processes and that these processes can be observed and evaluated bu scientific experimentation. These hypotheses proposed that inorganic matter changed into organic matter composed of complex carbon-containing molecules and that these in turn combined to form the first living cell. It is important to recognize that we will probably never know for sure how life on earth came to be, but it is interesting to speculate and examine the evidence related to this fundamental question.
The biogenesis concept (referred to as “directed panspermia”) receive renewed support when in 1969 in Murchison, Australia, a meteorite was found to contain amino acids and other complex organic molecules. In 1996 a meteorite from Antarctica was also analyzed. It has been known for many years that meteorites often contain organic molecules and this suggested that life may have existed elsewhere in the solar system. The chemical makeup of the Antarctica meteorite suggests that it was portion of the planet Mars, which was ejected from Mars, as a result of a collision between the planet and an asteroid. Analysis of the meteorite show the presence of complex organic molecules and small globules that resemble those found on earth that are though to be the result of the activity of ancient microorganisms. Because Mars currently has some water as ice and shows features that resemble dried up river systems, Mars may have had much more water in the past. For these reasons many believe it is reasonable to consider that life of a nature similar to that presently found on earth could have existed on mars.
The alternative view that life originated on the earth has also received support. Let us look at several lines of evidence.
the earth is the only planet in our solar system with a temperature range that allows for water to exist as a liquid on its surface, and water is the most common compound in most kinds living things.
analysis of atmosphere of other planets shows that they all lack oxygen. The oxygen in the earth’s atmosphere is the result of current biological activity. Therefore, before life on earth the atmosphere probably lacked oxygen.
experiments demonstrate that organic molecules can be generated in an atmosphere that lacks oxygen.
because it is assumed that all of the planets have been cooling off as they age, it is very likely that the earth was much hotter in the past. The large portions of the earth’s surface that are of volcanic origin strongly suggest a hotter past. There is also the likelihood that various large bodies collided with the earth early in its history and that they could have led to increased temperatures at least in the site of the collision.
recognition that there are distinct prokaryotic organisms that live in extreme environments of high temperature, high salinity, low PH, or the absence of oxygen suggests that they may have been adapted to life in a world that is very different from today’s earth. These kinds of organisms are found today in unusual locations such as hot springs and around thermal vents in the ocean floor and may be descendents of the first organisms formed on the primitive earth.
22.3 the “big bang” and the origin of the earth
As the astronomers and others look at the current stars and galaxies it can be observed that they are moving apart from one another. This and other evidence has led to the concept that our current universe began as a very dense mass of matter that had a great deal of energy. This dense mass of matter exploded in a big bang that resulted in the formation of atoms. According to this scientific theory the original universe consisted primarily of atoms of hydrogen and helium. The solar system theory proposed that the solar system was formed from a large cloud of gases that developed some 10 to 20 billion years ago. The simplest and most abundant gases would have been hydrogen and helium. A gravitational force was created by the collection of particles within this cloud that caused other particles to be pulled from the outer edges to the center. As particles collected into large bodies, gravity increased and more particles were attached to the bodies. Ultimately a central body was formed and several other bodies formed that moved around it. The sun consists primarily of hydrogen and helium atoms, which are being fused together to form larger atoms with the release of large amount of thermonuclear energy. Many scientists believe that earth—along with other planets, meteors, asteroids, and comets—was formed at least 4.6 Ba. A large amount of heat was generated as the particles became concentrated to form earth. Geologically, this is called the “hadean era”. The term Hadean means “hellish”, although not as hot as the sun, the material of earth formed a molten core that became encased by a thin outer crust as it cooled. In its early stages of formation, about 4 Ba, there may have been a considerable amount of volcanic activity on earth.
Physically, Earth was probably much different than it is today. Because the surface was hot , there was no water on the surface or in the atmosphere. In fact, the tremendous amount of heat probably prevented any atmosphere from forming. The gases associated with our present atmosphere (nitrogen, oxygen, carbon dioxide, and water vapor) were contained in the planet’s molten core. These hostile conditions (high temperature, lack of water, lack of atmosphere) on early Earth could not have supported any form of life similar to what we see today.
Over hundreds of millions of years, Earth is thought to have slowly changed. As it cooled, volcanic activity probably caused the release of water vapor, carbon dioxide, methane, ammonia, and hydrogen, and the early atmosphere was formed. These gases formed a reducing atmosphere—an atmosphere that did not contain molecules of oxygen. Any oxygen would have quickly combined with other atoms to form compounds, so a significant quantity of molecule oxygen would have been highly unlikely. Further cooling enabled the water vapor in the atmosphere to condense to form the oceans we see today.
22.4 steps needed to produce the life from inorganic materials
When we consider the nature of the simplest form of life today, we find that living things consists of an outer membrane that separates the cell from its surroundings, genetic material in the form of nucleic acids, and many kinds of enzymes that control the activities of the cell. Therefore, when we speculate about the origin of life from inorganic material it seems logical that several events or steps were necessary:
organic molecules must first be formed from inorganic molecules.
basic organic molecules form RNA that can serve as the genetic material and to catalyze other reactions.
RNA becomes self-replicating.
the organic RNA molecules must be collected together and segregated from other molecules by a membrane.
the control of protein synthesis must be taken over by RNA.
Protein became the catalysts (enzymes) of the cell.
DNA replaces RNA as the self-replicating genetic material of the cell.
ultimately these first cellular units must be able to reproduce more of themselves.
Formation of the first organic molecules
In the 1920s, a Russian biochemist, Alexander I. Oparin, and a British biologist, J. B. S. Haldane, working independently, proposed that the first organic molecules were formed spontaneously in the reducing atmosphere thought to be present on the early Earth. The molecules of water vapor, ammonia, methane, carbon dioxide, and nitrogen, and lightning, heat from volcanoes, and ultraviolet radiation furnished the energy needed for the synthesis of simple organic molecules. It is important to understand the significance of a reducing atmosphere to this theory. The absence of oxygen in the atmosphere would have allowed these organic molecules to remain and combine with one another. This does not happen today because organic molecules are either consumed by organisms or oxidized to simpler inorganic compounds in the atmosphere. Many kinds of air pollutants are organic molecules called hydrocarbons that eventually degrade into smaller molecules in the atmosphere. Unfortunately they participate in the formation of smog as they are broken down.
After these simple organic molecules were formed in the atmosphere, they probably would have been washed from the air and carried into the newly formed oceans by the rain. Here, the molecules could have reacted with one another to form the more complex molecules of simple sugars, amino acids, and nucleic acids. This accumulation is thought to have occurred over half a billion years, resulting in oceans that were a dilute organic soup. These simple organic molecules in the ocean served as the building materials for more complex organic macromolecules, such as complex carbohydrates, proteins, lipids, and nucleic acids. Recognize that all the ideas presented so far cannot be confirmed by direct observation because we cannot go back in time. However, several of these assumptions central to this theory of the origin of life have been laboratory tested.
In 1953 Stanley L. Miller conducted an experiment to test the idea that organic molecules could be synthesized in a reducing environment. Miller constructed a simple model of the early Earth’s atmosphere. In a glass apparatus he placed distilled water to represent the early oceans. Adding hydrogen, methane, and ammonia to the water to simulated the reducing atmosphere. Electrical sparks provided the energy needed to produce organic compounds. By heating parts of the apparatus and cooling others, he simulated the rains that are thought to have fallen into the early oceans. After a week of operation, he removed some of the water from the apparatus. When this water was analyzed, it was found to contain many simple organic compounds. Although Miller demonstrated nonbiological synthesis of simple organic molecules like amino acid and simple sugars, his results did not account for complex organic molecules like proteins and nuclei acids. However, other researchers produced some of the components of nucleic acid under similar primitive conditions.
Several ideas have been proposed for the concentration of simple organic molecules and their combination into macromolecules. The first hypothesis suggests that a portion of the early ocean could have been separated from the main ocean by geologic changes. The evaporation of water from this pool could have concentrated the molecules, which might have led to the manufacture of macromolecules by dehydration synthesis. Second, it has been proposed that freezing may have been the means of concentration. When a mixture of alcohol and water is placed in freezer, the water freezes solid and the alcohol becomes concentrated into a small portion of liquid. A similar process could have occurred on Earth’s early surface, resulting in the concentration of simple organic molecules. In this concentrated solution, dehydration synthesis in a reducing atmosphere could have occurred, resulting in the formation of macromolecules. A third theory proposes that clay particles may have a role in concentrating simple organic molecules. Small particles of clay have electrical charges that can attract and concentrate organic molecules like protein from a watery solution. Once the molecules became concentrated, it would have been easier for them to interact to form larger macromolecules.

Isolating organic molecules –coacervates and microspheres
Geologists and biologists typically measure the history of life by looking back from the present. Therefore, time scales are given in “years ago.” It has been estimated that the formation of simple organic molecules in the atmosphere began about 4 Ba and lasted approximately 1.5 billion years. The oldest known fossils of living cells are thought to have formed 3.5 Ba. Fossilized, photosynthetic bacteria have been found in geological formation called stromatolites on the coasts of South Africa and Western Australia. The question is , How do you get from the spontaneous formation of macromolecules to primitive cells in half a billion years?
Two hypotheses are proposed for the formation of prebionts, nonliving structures that led to the formation of the first living cells from which the more complex cells have today evolved. Oparin speculated that a prebionts consisted of carbohydrates, proteins, lipids, and nucleic acids that accumulated to form a coacervate. Such a structure could have consisted of a collection of organic macromolecules surrounded by a film of water molecules. This arrangement of water molecules, although not a membrane, could have ed as a physical barrier between the organic molecules and their surroundings. They could selectively take in materials from their surroundings and incorporate them into their structure.
Coacervates have been synthesized in the laboratory. They can selectively absorb chemicals from the surrounding water and incorporate them into their structure. Also, the chemicals within coacervates have a specific arrangement—they are not random collections of molecules. Some coacervates contain enzymes that direct a specific type of chemical reaction. Because they lack a definite membrane, no one claims coacervates are alive, but they do exhibit some lifelike traits: they are able to grow and divide if the environment is favorable.
An alternative hypothesis is that this early prebiotic cell structure could have been a microsphere or protocell. A microsphere is a nonliving collection of organic macromolecules with a double-layered outer boundary. Sidney Fox demonstrated the ability to build microspheres from proteinoids. Proteinoids are proteinlike structures consisting of branched chains of amino acids. Proteinoids are formed by the dehydration synthesis of amino acids at a temperature of 180 0C. Fox, from the University of Miami, showed that it is feasible to combine single amino acids into polymers of proteinoids. He also demonstrated the ability of to build microsphere from these proteinoids.
Microspheres can be formed when proteinoids are placed in boiling water and slowly allowed to cool. Some of the proteinoid material produces a double-boundary structure that encloses the microsphere. Although these walls do not contain lipids, they do exhibit some membranelike characteristics and suggest the structure of a cellular membrane. Microspheres swell or shrink depending on the osmotic potential in the surrounding solution. They also display a type of internal movement (streaming) similar to that exhibited by cells and contain some proteinoids that as enzymes. Using ATP as a source of energy, microspheres can direct the formation of polypeptides and nucleic acids. They can absorb material from the surrounding medium and form buds, which results in a second generation of microspheres. Given these characteristics, some investigators believe that microspheres can be considered protocells, the first living cells.
The laboratory synthesis of coacervates and microsphere helps us understand how the first primitive living cells might have developed. However, it leaves a large gap in our understanding because it does not explain how these first cells might have become the highly complex living cells we see today.

Meeting bolic needs—heterotrophs or autotrophs

Fossil evidence indicates that there primitive forms of life on Earth about 3.6 Ba. Regardless of how they developed, these first primitive sells would have needed a way to add new organic molecules to their structures as previously existing molecules were lost or destroyed. There are two ways to accomplish this. Heterotrophs capture organic molecules such as sugars, amino acids, or organic acids from their surroundings, which they used to make new molecules and provide themselves with a source of energy. Autotrophs use some external energy such as sunlight or the energy from inorganic chemical reactions to allow them to combine simple inorganic molecules like water and carbon dioxide to make new organic molecules. These new organic molecules can than be broken down at a later date to provide a source of energy.
Many scientists support the idea that the first living things produced on Earth were hetertrophs that lived off the organic molecules that would have been found in the oceans. Because he early heterotrophs are thought to have developed in a reducing atmosphere that lacked oxygen, they would have been of necessity anaerobic organisms; therefore they did not abtain the maximum amount of energy from the organic molecules they obtained from their environment. At first, this would not have been a problem. The organic molecules that had been accumulating in the ocean for millions of years served as an ample source of organic material for the heterotrophs. However, as the polulation of heterotrophs increased through reproduction, the supply of organic material would have been consumed faster than it was being spontaneously produced in the atmosphere. If there was no other source of organic compounds, the heterotrophs would have eventually exhausted their nutrient supply, and they would have become extinct.
Even though the early heterotrophs probably contained nucleic acids and were capable of producing enzymes that could regulate chemical reactions, the probably carried out a minimum of biochemical activity. There is evidence to suggest that a wide variety of compounds were present in the early oceans, some of which could have been used unchanged by the heterotrophs. There was no need for the heterotrophs to modify the compounds to meet their needs.
Those compounds that could be easily used by heterotrophs would have been the first to become depleted from the early environment. However, some of the heterotrophs may have contained a mutated form of nucleic acid, which allowed them to convert material that was not directly usable into compound that could be used. Mutations may have been common because the amount of ultraviolet light, one cause of mutations, would have been high. The absence of ozone in the upper atmosphere of the early Earth would have allowed high amounts of ultraviolet light to reach the Earth’s surface. Heterotrophs with such mutations could have survived, whereas those without it would have become extinct as the compounds they used for food became scarce. It has been suggested that through a series of mutations in the early heterotrophs, a more complex series of biochemicl reactions to convert ingestible chemicals into usable organic compounds.
As with many areas of science there are often differences of opinion. Although this heterotroph hypothsis for the origin of living things was the prevailing theory for many years, recent discoveries have caused many scientists to consider an alternative—that the first organism was an autotroph. Several kinds of information support this theory. Many kinds of very primitive prokaryotic organism, members of the Domain Archaea, were autotrophic and lived in extremely hostile environments. For this reason, they are referred as “extremophiles” (lovers of extremes). The nutrients they utilized were mostly likely CO2 ,CO, H2, H2S, N2, and S. the end products of their bolism were probably such compounds as H2SO4, CH4,and H2O. These organisms are found in hot springs like those found in Yellowstone National Park, Kamchatka, Russia, or near hot thermal vents—areas where hot mineral-rich water enters seawater from the deep ocean floor. They use inorganic chemical reactions as a source of energy to allow them to synthesize organic molecules from inorganic components. The fact that many of these organism live in very hot environments suggests that they may have originated on an Earth that was much hotter than it is currently. There is much evidence that the earth was a much hotter place in the past. If the first organisms were autotrophs there could have been subsequent evolution of a variety of kinds of cells, both autotrophic and heterotrophic, that could have led to the diversity of different prokaryotic cell seen today in the Domain Eubacteria and Archaea.

Reproduction and the origin of the genetic material
The reproduction of most current organisms involves the replication of DNA and the distribution of the copied DNA to subsequent cells. (those that do not use DNA use RNA as their genetic material.) DNA is responsible for the manufacture of RNA, which subsequently leads to the manufacture of proteins. This is the central dogma of modern molecular biology. However, it is difficult to see how this complicated sequence of events, which involves many steps and the assistance of several enzymes, could have been generated spontaneously, so scientists have looked for simple systems that could have led to the DNA system we see today.
   Science works simultaneous on several fronts. Scientists involved in studying the structure and of viruses discovered that many viruses do not contain DNA but store their genetic information in the structure of RNA. In order for these RNA-viruses to reproduce, they must enter a cell and have their RNA reverse-transcribed into DNA, which the host cell translates to manufacture new virus protein and RNA.
Other scientists who study viral disease find that it is difficult to develop vaccines for many viral disease because their genetic material easily mutates. Because of this, researchers have been studying the nature of viral DNA or RNA to see what cause the high rate of mutation. This has led others to explore the RNA viruses and ask the question: can RNA replicated itself without DNA? This is important question, because if RNA can replicate itself it would have all the properties necessary to serve as genetic material. It could store information, translate information intro protein structure, mutate, and make copies of itself.
Other research about the nature of RNA provides interesting food for thought. RNA can be assembled from simple subunits that could have been present on the early Earth. Scientists have also shown that RNA molecules are able to make copies of themselves without the need for enzymes, and they can do so without being inside cells. These molecules have been called ribozymes. This new evidence suggests that RNA may have been the first genetic material and help solve one of the problems associated with the origin of life: how genetic information was store in these primitive life-forms. Because RNA is a much simpler molecule than DNA and can make copies of itself without the help of enzymes, perhaps it was the first genetic material. Once a primitive life-form had the ability to cope its genetic material it would be able to reproduce. Reproduction is one of the most fundamental characteristics of living things.
As a result of this discussion you should understand that we do not know how life on Earth originated. Scientists look at many kinds of evidence and continue to explore new avenues of research. So we currently have three competing theories for the origin of life on Earth:
life arrived from some extraterrestrial source (directed panspermial/biogenesis).
life originated on Earth as a heterotroph (spontaneous generation).
life originated on Earth as an autotroph (spontaneous generation).

22.5 major evoluationary changes in the nature of living things

Once living things existed and had a genetic material that stored information but was changeable, living things could have proliferated into a variety of kinds that were adapted to specific environmental conditions. Remember that the earth has not been static but has been changing as a result of its cooling, volcanic activity, and encounters with asteroids. In addition, the organisms have had an impact on the way in which the Earth has developed. Regardless of the way in which life originated on earth, there have been several major events in the subsequent evolution of living things.

The development of oxidizing atmosphere

Ever since its formation, earth has undergone constant change. In the beginning, it was too hot to support an atmosphere. Later, as it cooled and as gases escaped from volcanoes, a reducing atmosphere (lacking oxygen) was the likely to have been formed. The early life-forms would have lived in this reducing atmosphere. However, today we have an oxidizing atomosphere and most organisms use this oxygen as a way to extract energy from organic molecules through a process of aerobic respiration. But what caused the atmosphere to change? Today it is clear that the oxygen in our atmosphere is the result of the process of photosynthesis. Prokaryotic cyanobacteria are the simplest organisms that are able to photosysthesize so it seems logical that the first organism could have accumulated many mutations over time that could have resulted in photosysthetic autotrophs. One of the waste products of the process of photosynthesis is molecular oxygen. This would have been a significant change because it would have led to the development of an oxidizing atmosphere, which contains molecular oxygen. The development of an oxidizing atmosphere created an environment unsuitable for the formation of organic molecules. Organic molecules tend to break down when oxygen is present. The presence of oxygen in the atmosphere would make it impossible for life to spontaneously originate in the manner described earlier in this chapter because an oxidizing atmosphere would not allow the accumulation of organic molecules in the seas. However, new life are generated through reproduction, and new kinds of life are generated through mutation and evolution. The presence of oxygen in the atmosphere had one other important outcome: it opened the door for the evolution of aerobic organisms.
It appears that an oxidizing atmosphere began to develop about 2 Ba. Although various chemical reactions released small amounts of molecular oxygen into the atmosphere, it was photosysthesis that generated most of the oxygen. The oxygen molecules also reacted with one another to form ozone. Ozone collected in the upper atmosphere and acted as a screen to prevent most of the ultraviolet light from reaching Earth’s surface. The reduction of ultraviolet light diminished the spontaneous formation of complex organic molecules. It also reduced the number of mutations in cells. In an oxidizing atmosphere, it was no longer possible for organic molecules to accumulate over millions of years to be later incorporated into living material.
The appearance of oxygen in the atmosphere also allowed for evolution of aerobic respiration. Because the first heterotrophs were of necessity anaerobic organism, they did not derive amounts of energy from the organic materials available as food. With the evolution of aerobic heterotrophs, there could be a much more efficient conversion of food into usable energy. Aerobic organisms would have a significant advantage over anaerobic organisms: they could use the newly generated oxygen as a final hydrogen acceptor and, therefore, generate many more ATPs from the food molecules they consumed.

The establishment of three major Domain of life
In 1977 Carl Woese published the idea that the “bacteria”, which had been considered a group of similar organisms, were really made up of two very different kinds of organisms: the Eubacteria and Archaea. Furthermore the Archaea shared some characteristics with eukaryotic organisms. Subsequent investigations have supported these ideas and led to an entirely different way of looking at the classification and evolution of living things. Although biologists have traditionally divided organisms into kingdoms based on their structure and , it was very difficult to do this with microscopic organisms. With the newly developed ability to decode the sequence of nucleic acids, it became possible to look at the genetic nature of organisms without being confused by their external structures. Woese studied the sequences of ribosomal RNA and compared similarities and differences. As a result of his studies and those of many others a new concept of the relationships between various kinds of organisms has emerged.
   The three main kinds of living things, enbacteria, archaea, and eucarya, have been labeled “domains.” Within each domain there are several kingdoms. In the euncarya there are four kingdoms that we already recognize: animalia, plantae, fungi, protista. However, previously all of the eubacteria and archaea have been lumped into the same kingdom: prokaryote. It has become clear that there are great differences between the enbacteria and archaea, and within each of these groups there are greater differences than are found among the other four kingdoms (animalia, plantae, fungi, and protista).
   This new picture of living things requires us to reorganize our thinking. It appears that the oldest organisms may have been bacteria that are able to live in hot situations and that they gave rise to the archaea, many of whom still require extreme environments. Perhaps most startling is the idea that the archaea and eucarya share many characteristics suggesting that they are more closely related to each other than either is to the enbacteria.
It appears that each domain developed specific abilities. The archaea are primarily organisms that use inorganic chemical reactions to generate the energy they need to make organic matter. Often these reactions result in the production of methane. These organisms are known as methanogens. Others use sulfur and produce hydrogen sulfide. Most of these organisms are found in extreme environments such as hot springs or in extremely salty or acid environments.
The eubacteria developed many different bolic abilities. Today many are able to use organic molecules as a source of energy, some are able to carry on photosynthesis, and still others are able to get energy from inorganic chemical reactions similar to archaea.
The eucarya are the most familiar and appear to have exploited the bolic abilities of other organisms by incorporating them into their own structure. Chloroplasts and mitochondria are both bacterialike structures found inside eukaryotic cells. Table 22.1 summarizes the major characteristics of these three Domains.

The origin of eukaryotic cells
The earliest fossils appear to be similar in structure to that of present-day bacteria. Therefore it is likely that the early heterotrophs and autotrophs were probably simple one-celled organisms like bacteria. They were prokaryotes that lacked nuclear membranes and other organelles, such as mitochondria, an endoplasmic reticulum, chloroplast, and a Golgi apparatus. Present-day bacteria and archaea are prokaryotes, all other forms of life are eukaryotes, which posess a nuclear membrane and other membranous organelles.
Biologists generally believe that the enkaryotes evolved from the prokaryotes. The endosymbiotic theory attempts to explain this evolution. This theory suggests that present-day eukaryotic cells evolved from the combining of several different types of primitive prokaryotic cells. It is thought that some organelles found in enkaryotic cells may have originated as free-living prokaryotes. For example, because mitochondria and chloroplasts contain bacteria-like DNA and ribsomes, control their own reproduction, and synthesize their own enzymes, it has been suggested that they were once free-living prokaryotes. These bacteria cells could have established a symbiotic relationship with another primitive nuclear membrane-containing cell type. When this theory was first suggested it met with a great deal of criticism. However, continuing research has uncovered several other instances of the probable joining of two different prokaryotic cells to form one.
If these cells adapted to one another and were able to survive and reproduce better as a team, it is possible that this relationship may have evolved into present-day eukaryotic cells. If this relationship had included only a nuclear membrane-containing cell and aerobic bacteria, the newly evolved cell would have been similar to present-day heterotrophic protozoa, fungi, and animal cells. If this relationship had included both aerobic bacteria and photosynthetic bacteria,  the newly formed cell would have been similar to present-day autotrophic algae and plant cells. In addition it is likely that endosymbiosis occurred among eukaryotic organisms as well. Several kinds of enkaryotic red and brown algae contain chloroplastlike structures that appear to have originated as free-living eukaryotic cells.
Regardless of the type of cell, or whether the organisms are heterotrophic or autotrophic, all organisms have a common basis. DNA is the universal genetic material; protein serves as structural material and enzymes; and ATP is the source of energy. Although there is a wide variety of organisms, they all are built from the same basic molecular building blocks. Therefore, it is probable that all life derived from a single origin and that the variety of living things seen today evolved from the first protocells.
Let us return for moment to the question that perplexed early scientists and caused the controversies surrounding the opposing theories of spontaneous generation and biogenesis. From our modern perspective we can see that all life we experience comes into being as a result of reproduction. Life is generated from other living things, the process of biogenesis. However, reproduction does not answer the question: where did life come from in the first place? We can speculate, test hypothesis, and discuss various possibilities, but we will probably never know for sure. Life either always was or it started at some point in the past. If it started, then spontaneous generation of some type had to occur at least once, but it is not happening today.
22.6 evolutionary time line
A geological time chart shows a chronological history of living organisms based on the fossil record. The largest geological time unit called eons. From earliest to most recent, the geological eras of the Precambrian eon are the Hadean, Archaean, and proterozoic. The Phanerozoic eon is divided into the eras Paleozoic, Mesozoic, and Cenozoic. Each of these eras is subdivided into smaller time units called periods. For example, Jurassic is a period of the Mesozoic Era that began 180 million years ago.
Recent evidence suggests that prokaryotic cell types most likely came into existence approximately 3800 to 3700 million years ago during the Archaean Era of the Precambrian Eon. This was just prior to the development of the prokaryotic life-forms that are members of the Domain Archaea. The photosynthetic eubacterial cyanobacteria are though to be have been responsible for the production of molecules oxygen that began to accumulate in the atmosphere and make conditions favorable fro the evolution of other types of cells. Members of the Archaea are often referred as extremophiles since they live in extreme environments. This includes environments that are extremely acid, hot, or otherwise chemically inhostable to other life forms. To date, the bacterium, Pyrolobus fumarii, bas been identified a the most extreme thermophile growing at 113 0C at sea bottom! The first members of the domain eucarya, the eukaryotic organisms, appeared approximately 1.8 billion years ago.
The part of the Earth’s history dominated by unicellular organisms is generally referred as the Precambrian Eon. There is little fossil record of Precambrian unicellular life, although it comprises a span of time much greater than the entire history of multicellular plants and animals. The first multicellular organisms appeared 700 millions years ago at the end of the Precambrian eon. During the Cambrian Period of the Paleozoic Era, an explosiong of multicellular organisms occurred. Marine invertebrates were abundant, with individuals from most present-day phyla existing at that time.  Phylum
Several other “explosions,” or adaptive radiations, followed. Note on the geological time chart that a different major form of vegetation dominated each era. The Paleozoic Era was dominated by nonvascular and primitive vascular plants; the Mesozoic era by cone-bearing evergreens; and the Cenozoic by the flowering plants with which we are most familiar dominated the Paleozoic era. Likewise, many periods are associated with specific animal groups. Among vertebrates, the Devonian period is considered the age of fishes and the Pennsylvanian period the age of amphibians. The Mesozoic era is considered the age of mammals. In each instance, dominance of a particular animal group resulted from adaptive radiation events.
Amphibians, for example, most likely evolved from a lobe-finned fish of the Devonian period. This organism possessed two important adaptations: lungs and paired lobed fins that allowed the organism to pull itself onto land and travel to new water holes during times of drought. Selective pressures resulted in fins evolving into legs and the first amphibian came into being. During this lengthy time, landmasses were colonized by vegetation but only a few types of animals moved onto the land. The first vertebrates to spend part of their lives on land found a variety of unexploited niches resulting in the rapid evolution of new amphibian species and their dominance during the Pennsylvanian period.
For 40 million years, amphibians were the only vertebrate animals on land. During this time, mutations continued to occur, and valuable modifications were passed on to future generations that eventually led to the development of reptiles. One change allowed the male to deposit sperm directly within the female. Because the sperm could directly enter the female and remain in a moist interior, it was no longer necessary for the animals to return to the water to mate, as amphibians still must do. However, developing young still required a moist environment for early growth. A second modification, the amniotic egg, protects the developing young from injury and dehydration while allowing for the exchange of gases with the external environment. A third adaptation, the development of protective scales and relatively impermeable skin, protected reptiles from dehydration. With these adaptations, reptiles were able to outcompete amphibians in most terrestrial environments. Amphibians that did survive were the ancestors of present-day frogs, toads, and salamanders. With extensive adaptive radiation, reptiles took to the land, sea, and air. A particular successful group of reptiles was the dinosaurs. The length of time that dinosaurs dominated the earth, more than 100 millions years, was greater than the length of time from their extinction to the present.
As reptiles diversified, some developed characteristics common to other classes of vertebrates found today, such as warm-bloodedness, feathers, and hair. Warm-blooded reptiles with scales modified as feathers for insulation eventually evolved into organisms capable of flight. Through natural selection, reptilian characteristics were slowly eliminated and characteristics typical of today’s modern birds (multiple adaptations to flight, keen sense, and complex behavioral instincts) developed. Archaeopteyx, the first bird, had characteristics typical of both birds and reptiles.
Also evolving from reptiles were the mammals. The first reptiles with mammalian characteristics appeared in the Permian period, although the first true mammals did not appear until the Triassic period. These organisms remained relatively small in number and size until after the mass extinction of the reptiles. Extinctions opened many niches and allowed for the subsequent adaptive radiation of mammals. As with the other adaptive radiations, mammals possessed unique characteristics that made them better adapted to the changing environment; the characteristics include insulating hair, constant body temperature, and internal development of young. Figure 22.14 summarizes the hypothetical evolutionary time line.

Probably nothing interests people more than sex and sexuality. By sexuality, we mean all the factors that contribute to one's female or male nature. These include the structure and of the sex organs, the behaviors that involve these structures, psychological components,  the role culture plays in manipulating our sexual behavior. Males and females have different behavior patterns for a variety of reasons. Some behavioral differences are learned (patterns of dress, use of facial makeup), whereas others appear to be less dependent on culture (degree of aggressiveness, frequency of sexual thoughts). We have an intense interest in the facts about our own sexual nature and the sexual behavior of members of the opposite sex and that of peoples of other cultures.

    There are several different ways to look at human sexuality. The behavioral sciences tend to focus on the behaviors associated with being male and female and what is considered appropriate and inappropriate sexual behavior. Sex is considered a strong drive, appetite, or urge by psychologists. They describe the sex drive as a basic impulse to satisfy a biological, social, or psychological need. Other social scientists (sociologist, cultural anthropologists) are interested in sexual behavior as it occurs in different cultures and subcultures. When a variety of cultures are examined, It becomes very difficult to classify various kinds of sexual behavior as normal or abnormal. What is considered abnormal in one culture may be normal in another. For example, public nudity is considered abnormal in many cultures but not in others.

   The sexual behavior on nonhuman animals has been studied by biologists for centuries. Biologists have long considered the of sex and sexuality in light of its to the population or species. Sexual reproduction results in new combinations of genes that are important in the process of natural selection. Many biologists today are attempting to look at human sexual bebavior from an evolutionary perspective and speculate on why certain sexual behaviors are common in humans. The behaviors of courtship, mating, rearing of the young, and the division of labor between the sexes are complex in all social animals, including humans. These are demonstrated in the elaborate social behaviors surrounding mate seletion and the establishment of families. It is difficult to draw the line between the biological development of seuality and the social establishment of customs related to the sexual aspects of human life. However, the biological mechanism that determines whether an individual will develop into a female ro male has been well documented.

 21.2 chromosomal determination of sex

When a human egg or sperm cell is produced, it contains 23 chromosomes. Twenty-two of these are autosomes that carry most of the genetic information used by the organism. The other chromosome is a sex-determining chromosome. There are two kinds of sex-determining chromosomes: the X chromosome and the Y chromosome. The two sex-determining chromosomes, X and Y, do not carry equivalent amounts of information, nor do they have equal s. X chromosomes carry typical genetic information about the production of specific proteins in addition to their in determining sex. For example, the X chromosome carries information on blood clotting, color vision, and many other characteristics. The Y chromosome, however, appears to be primarily concerned with determining male sexual differentiation and has few other genes on it.

    When a human sperm cell is produced, it carries 22 autosomes and a sex-determining chromosome. Unlike eggs, which always carry an X chromosome, half the sperm cells carry an X chromosome and the other half carry a Y chromosone. If an X-carrying sperm cell fertilizes with X-containing egg cell, the resultant embryo will develop into a female. A typical human female has an X chromosome from each parent. If a Y-carrying sperm cell fertilizes the egg, a male embryo develops. It is the presence of absence of the y chromosome that determines the sex of the developing individual.

   Evidence that the y chromosome controls male development comes as a result of studying individuals who have an abnormal number of chromosomes. As abnormal meiotic division that results in sex cells with too many or too few chromosomes is called nondisjunction. If nondisjunction affects the x and y chromosomes, a gamete might be produced that has only 22 chromosomes and lacks a sex-determining chromosome, or it might have 24, with two sex-determining chromosomes. If a cell with too few or too many sex chromosomes is fertilized, an abnormal embryo develops. If a normal egg cell is fertilized by a sperm cell with no sex chromosome, the offspring will have only one X chromosome. These people are designated as XO. They develop a collection of characteristics known as Turner's syndrome person. An individual with this condition is female, is short for her age, and fails to mature sexually, resulting in sterility. In addition, she may have a thichened neck (termed webbing), hearing impairment, and some abnormalities in the cardiovascular system. When the condition is diagnosed, some of the physical conditions can be modified with treatment. Treatment involves the use of growth-stimulating hormone to increase growth rate and the use of female sex hormones to stimulate seual development, although sterility is not corrected.

  An individual who has XXY chromosomes is basically male. This genetic anomaly is termed Klinefelter's syndrome, and the symptoms include sterility because of small testes that do not usually produce viable sperm, lack of facial hair, and occasional breast tissue development. These persons are also more likely than most to experience difficulty with language development. Although they are sterile, men with this condition have normal sexual . These characteristics vary greatly in degree and many men are diagnosed only after they undergo testing to determine why they are infertile. This condition is present in about 1 in 500 men. Treatment may involve breast-reduction surgery in males who have significant breast development and male hormone therapy.

   Because both conditions involve abnormal numbers of X or Y chromosomes, they provide strong evidence that these chromosomes are involved in determining sexual development. The early embryo resulting from fertilization and cell devision is neither male or female but becomes female or male later in development--based on the sex-determining chromosomes that control the specialization of the cells of the undeveloped, embryonic gonads into female ovaries or male testes. This specialization of embryonic cells is termed differentiation. The embryonic gonads begin to differentiate into testes about seven weeks after conception if the y chromosome is present. The y chromosome seems to control this differentiation process in males because the gonads do not differentiate into female sex organs until later, and then only if two X chromosomes are present. It is the absence of the Y chromosome that determines female sexual differentiation

  researchers were interested in how females, with two X chromosomes, handle the double dose of genetic material in comparison to males, who have only one X chromosome. M.L. Barr discovered that a darkly staining body was generally present in female cells but was not present in male cells. It was postulated, and has since been confirmed, that this structure is an X choromosome that is largely nonal. Therefore, female cells have only one dose of X-chromosome genetic information that is al; the other x chromosome coils up tightly and does not direct the manufacture of proteins. The one X chromosome of the male s as expected, and the Y chromosome directs only male-determining activities. The tightly coiled structure in the cells of female mammals is called Bar body after its discoverer

21.3 male and female fetal development

Development of embryonic gonads begins very early during fetal growth. First, a group of cells begins to differentiate into primitive gonads at about week 5. by week 6 or 7 if a Y chromosome is present, a gene product from the chromosome will begin the differentiation of these gonads into testes; they will develop into ovaries beginning about week 12 if two x chromosomes are present.

   As soon as the gonad has differentiated into an embryonic testis at about week 8, it begins to produce testosterone. The presence of testosterone results in the differentiation of male sexual anatomy and the absence of testosterone results in the differentiation into female sexual anatomy.

   IN normal males, at about the seventh month of gestation, the testes move from a position in the abdomianl cavity to the external sac, called the scrotum, via an opening called the inguinal canal. This canal closes off but continues to be a weakened area in the abdominal wall and may rupture later in life. This can happen when strain (e.g., from improperly lifting heavy objects) causes a portion of the intestine to push through the inguinal canal into the scrotum. this condition is known as an inguinal hernia.

   Occasionally the testes do not descend and a condition known as cryptorchidism develops. Sometimes the descent occurs during puberty; if not, there is an increased incidence of testicular cancer. because of this increased risk, surgery is performed that allows the undescended testes to descend to their normal positions in the scrotum. the retention of the testes in the abdomen results in sterility because normal sperm cell development can not coccur in a very warm environment and the temperature in the abdomen is higher than the temperature in the scrotum. Normally the temperature of the testes is very carefully regulated by muscles that control their distance from the body. Physicians have even diagnosed cases of male infertility as being caused by tight-fitting pants that hold the testes so close to the body that the temperature increase interferes with normal sperm development.

21.4 Sexual maturation of young adults

Following birth, sexuality plays only a small part in in physical development for several years. Culture and evironment shape the responses that the individual will come to recognize as a normal behavior. During puberty, normally between 12 and 14 years of age, increased production of sex hormones causes major changes as the individual reaches sexual maturity. Generally famales reach puberty six months to a year before males. After puberty, humans are sexually mature and have the capacity to produce offspring.

The maturation of females

female children typically begin to produce quantities of sex hormones from the hypothalamus, pituitary gland, ovaries and adrenal glands at 8 to 13 years of age. This marks the onset of puberty. The hypothalamus controls the ing of many other glands throughout the body, including the pituitary gland. at puberty the hypothalamus begins to release a hormone known as gonadotrophin-releasing hormone which stimulates the pituitary to release luteining hormone and follicle-stimulating hormone. Increased levels of FSH stimulate the development of follicles, saclike structures that produce oocytes in the ovary, and the increased luteining hormone stimulates the ovary to produce larger quantities of estrogens. the increasing supply of estrogen is responsible for the many changes in sexual development that can be noted at this time. These changes include breast growth, changes in the walls of the uretus and vagina, increased blood supply to the clitoris, and changes in the pelvic bone structure.

    Estrogen also stimulates the female adrenal gland to produce androgens, male sex hormones. The androgens are responsible for the production of pubic hair and they seem to have an influence on the female sex drive. The adrenal gland secretions may also be involved in the development of acne. Those features that are not primarily involved in sexual reproduction but are characteristic of a sex are called secondary sexual characteristics. In women, the distribution of body hair, patterns of fat deposits,and a higher voice are examples.

   A major development during this time is the establishment of the menstrual cycle. This involves the periodic growth and shedding of the lining of the uretus. These changes are under the control of a number of hormones produced by the pituitary and ovaries. The ovaries are stimulated to release their hormones by the pituitary gland, which is in turn influenced by the ovarian hormones. Both follicle-stimulating hormone and luteinizing hormone are produced by the pituitary gland. FSH causes the maturation and development of the ovaries, and LH is important in causing ovulation and converting the ruptured follicle into a structure known as the corpus luteum that produces the hormone, progesterone, which is important in maintaining the lining of the uterus. Changes in the levels of progesterone result in a periodic buildup and shedding of the lining of the uterus known as the menstrual cycle. Table 21.1 summarizes the activities of these various hormones. Associated with the menstrual cycle is the periodic release of sex cells from the surface of the ovary, called ovulation. Initially, these two cycles, mentruation and ovulation, may be irregular, which is normal during puberty. Eventually, hormone production becomes regulated so that ovulation and menstruation take place on a regular monthly basis in most women, althrough normal cycles may vary from 21 to 45 days.

  As girls progress through puberty curiosity about the changing female body form and new feelings leads to self-investigation. Studies have shown that sexual activity such as manipulation of the clitoris, which causes pleasurable sensations, is performed by a large percentage of young women. Self-stimualtion, frequently to orgasm, is a common result. This stimulation is termed masturbation, and it should be stressed that it is considered a normal part of sexual development. Orgasm is a complex response to mental and physical stimulation that causes rhythmic contractions of the muscles of the reproductive organs and an intense frenzy of excitement.

The maturation of males

Males typically reach puberty about two years later than females, but puberty in males also begins with a change in hormone levels. At puberty the hypothalamus release increased amounts of gonadotropin-releasing hormone, resulting in increased levels of follicle-stimulating hormone and luteinizing hormone. These are the same changes that occur in female development. Luteinizing hormone is often called interstitial cell-stimulating hormone in males. ICSH stimulates the testes to produce testosterone, the primary sex hormone in males. The testosterone produced by the embryonic testes caused the differentiation of internal and external gential anatomy in the male embryo. At puberty the increased amount of testosterone is responsible for the development of male secondary sexual characteristics and is also important in the maturation and production of sperm.

   The major changes during puberty include growth of the testes and scrotum, pubic-hair development, and increased size of the penis. Secondary sex characteristics begin to become apparent at age 13 or 14. Facial hair, underarm hair, and chest hair are some of the most obvious. The male voice changes as the larynx begins to change shape. body contours also change, and a growth spurt increases height. In addition, the proportion of the body that is muscle increases and the proportion of body fat decreases. AT this time, a boy's body begins to take on the characteristic adult male shape, with broader shoulders and heavier muscles.

  In addition to these external changes, increased testosterone causes the production of seminal fluid by the seminal vesicles, prostate gland, and the bulbourethral glands.
FSH stimulats the production of sperm cells. The release of sperm cells and seminal fluid begins during puberty and  is termed ejaculation. This release is generally accompanied by the pleasurable sensations of orgasm. The sensations associated with ejaculation may lead to self-stimulation, or masturbation. Masturbation is a common and normal activity as a boy goes through puberty. Studies of sexual behavior have shown that nearly all men masturbation at some time during their lives.

21.5 spermatogenesis

One of the biological reasons for sexual activity is the production of offspring. The process of producing gametes includes meiosis and is called gametogenesis. The term spermatogenesis is used to describe gametogenesis that takes place in the testes of males. The two bean-shaped testes are composed of many small sperm-producing tubes, or seminiferous tubules, and collecting ducts that store sperm. These are held together by a thin covering membrane. The seminiferous tubules join together and eventually become the epidedymis, a long, narrow convoluted tube in which sperm cells are stored and mature before ejaculation.

    Leading from the edididymis is the vas deferens, or sperm duct; this empties into the urethra, which conducts the sperm out of the body through the penis. Before puberty, the seminferous tubules are packed solid with diploid cells called spermatogonia. these cells, which are found just inside the tubule wall, undergo mitosis and produce more spermatogonia. begining about age 11, some of the spermatogonia specialize and begin the process of meiosis, whereas others continue to divide by mitosis, assuring a constant and continuous supply of spermatogonia. Once spermatogenesis begins, the seminiferous tubules become hollow and can transport the mature sperm.

    Spermatogenesis involves several steps.Some of the spermatogonia in the walls of the seminiferous tubules differentiate and enlarge to become primary spermatocytes.  These diploid cells undergo the first meiotic division, which produces two haploid secondary spermatocytes. The secondary spermatocytes go through the second meiotic division, resulting in four haploid spermatids, which lose much of their cytoplasm and develop long tails. These cells are then known as sperm. The sperm have only a small amount of food reserves. Therefore, once they are released and become active swimmers, they live no more than 72 hours. However, if the sperm are placed in a special protective solution, the temperature can be lowered drastically to -196 C. Under these conditions the sperm freeze, become deactivated, and can live for years outside the testes. this has lead to the development of sperm banks. Artificial insemination of cattle, horses, and other demesticated animals with sperm from sperm banks is common. Human artificial insemination is much less common and is usually considered only by couples with fertility problems.

    Spermatogenesis in human males takes place continuously throughout a male's reproductive life, although the number of sperm produced decreases as a man ages. Sperm counts can be taken and  used to determine the probability of successful fertilization. For reasons not totally understood,a man must be able to release at least 100 million sperm at one insemination to be fertile. It appears that enzymes in the head of sperm are needed to digest through mucus and protein found in the female reproductive tract. millions of sperm contribute in this way to the process of fertilization, but only one is involved in fertilizing the egg. A healthy male probably releases about 300 million sperm with each ejaculation (although the numbers of sperm per ejaculate may be reduced with frequent ejaculation) during sexual intercourse, also known as coitus or copulation.

21.6 Oogenesis

The term oogenesis refers to the production of egg cells. This process starts during prenatal development of the ovary, when diploid oogenia cease dividing by mitosis and enlarge to become primary oocytes. All of the primary oocytes that a woman will ever have are already formed prior to her birth. AT this time they number approximately 2 million, but that number is reduced by cell death to between 300,000 to 400,000 cells by the time of puberty. Oogenesis halts at this point and all the primary oocytes remain just under the surface of the ovary.

   Primary oocytes begin to undergo meiosis in the normal manner at puberty. At puberty and on a regular basis thereafter, the sex hormones stimulate a primary oocyte to continue its maturaton process, and it goes through the first meiotic division. But in telophase I, the two cells that form receive unequal portions of cytoplasm. You might think of it as a lopsided division. The smaller of the two cells is called a polar body, ant the larger haploid cell is the secondary oocyte. The other primary oocytes remain in the ovary. Ovulation begins when the soon-to-be-released secondary oocyte encased in a saclike structure known as a follicle, grows and moves near the surface of the ovary. When this maturation is complete, the follicle erupts and the secondary oocytes is released. It is swept into the oviduct by ciliated cells and travels toward the uterus. Because of the action of the luteinizing hormone, the follicle from which the oocyte ovulated develops into a glandlike structure, the corpus luteum, which produces hormones that prevent the release of other secondary oocytes.

   If the secondary oocyte is fertilized, it completes meiosis by proceeding through meiosis II with the sperm DNA inside. During the second meiotic division, the secondary oocyte again divides unevenly, so that a second polar body forms. None of the polar bodies survives; therefore, only one large secondary oocyte is produced from each primary  oocyte that begins oogenesis. If the cell is not fertilized, the secondary oocytes passes through the vagina to the outside during menstruation. During her lifetime, a female releases about 300 to 500 secondary oocytes. Obviously, few of these cells are fertilized.

   One of the characteristics to note here is the relative age of the sex cells. In males, sperm production is continuous throughout life. Sperm do not remain in the tubes of the male reproductive system for very long. They are either released shortly after they form or die and are harmlessly absorbed. In females, meiosis begins before birth, but the oogenesis process is not completed, and the cell is not released for many years. A secondary oocyte released when a woman is 37 years old began meiosis 37 years before! during that time, the cell was exposed to many influences, a number of which may have demaged the DNA or interfered with the meiotic process. This has been postulated as a possible reason for the increased incidence of nondisjunction in older women. Such alterations are less likely to occur in males because new gametes are being produced continuously. Also, defective sperm appear to be much less likely to be involved in fertilization.

   Hormones control the cycle of changes in breast tissue, in the ovaries, and in the uretus. IN particular, estrogen and progestrone stimulate milk production by the breasts and cause the lining of the uretus to become thicker and more vascularized prior to the release of the secondary oocyte. This ensures that if the secondary oocyte becomes fertilized, the resultant embryo will be able to attach itself to the wall of the uterus and receive nourishment. If the cell is not fertilized, the lining of the uretus is shed. This is known as mentruation, menstrual flow, the menses, or a period. Once the wall of the uterus has been shed, it begins to build up again. As noted previously, this continual building up and shedding of the wall of the uretus is known as the menstrual cycle.

   The activities of the ovulatory cycle and the menstrual cycle are coordinated. During the first part of the menstrual cycle, increased amounts of FSH cause the follicle to increase in size. Simultaneously, the follicle secretes increased amounts of estrogen that cause the lining of the uterus to increase in thickness. When ovulation occurs, the remains of the follicle is converted into a corpus luteum by the action of LH. The corpus luteum begins to secrete progestrone and the nature of teh uterine lining changes by becoming more vascularized. This is choreographed so that if an embryo arrives in the uterus shortly after ovulation, it meets with a uterine lining prepared to accept it. If pregancy does not occur, the corpus luteum degenerates, resulting in a reduction in the amount of progesterone needed to maintain the lining of the uterus, and the lining is shed.
  

21.9 contraception

throughout history people have tried various methods of conception control. In ancient times, conception control was encouraged during times of food shortage or when tribes were on the move from one area to another in search of a new home. Writing as early as 1500 BC indicates that the Egypitians used a form of shrub to prevent fertilization. this may sound primitive, but we use the same basic principle today to destroy sperm in the vagina.

  contraceptive jellies and foams make the environment of the vagina more acidic, which diminish the sperm's chances of survival. The spermicidal foam or jelly is placed in the vagina before intercourse. When the sperm make contact with the acidic environment, they stop swimming and soon die. aerosol foams are an effective method of conception control, but interfering with the hormone regulation is more effective.

   The first successful method of hormonal control was "the pill". One of the newest methods of conception control also involves hormones. The hormones are contained within small rods or capsules, which are placed under a woman's skin. These rods, when properly implanted, slowly release hormones and prevent the maturation and release of oocytes from the follicle. The major advantage of the implant is its convenience. Once the implant has been inserted, the woman can forget about contraceptive protection for several years. If she wants to become pregnant, the implants are removed and her normal menstral and ovulation cycles return over a period of weeks.

   Killing sperm or preventing ovulation are not the only methods of preventing conception. Any method that prevents the sperm from reaching the oocyte prevents conception. One method is to avoid intercourse during those times of the month when a secondary oocyte may be present. This is known as the rhythm method of conception control. Although at first glance it appears to be the simplest and least expensive, determining jus when a secondary oocyte is likely to be present can be very difficult. A woman with a regular 28-day menstrual cycle will typically ovulate about 14 days before the onset of the next menstrual flow. in order to avoid pregnancy, couples need to abstain from intercourse a few days before and after this date. However, if a woman has an irregular menstrual cycle, there may be only a few days each month for intercourse without the possibility of pregnancy. IN addition to calculating safe days based on the length of the mentrual cycle, a woman can better estimate the time of ovulation by keeping a record of changes in her body temperature and viginal PH. Both changes are tied to the mentrual cycle and can therefore help a woman predict ovulation. In particular, at about time of ovulation a woman has a slight rise in boyd temperature--less than 1 C. Thus, one should use an extremely sensitive thermometer. A digital-readout thermometer on the market spells out the word yes or no.

21.11 sexual in the elderly
although there is a great deal of variation, somewhere around the age of 50, a woman’s hormonal balance begins to change because of changes in the production of hormones by the ovaries. As this time, the menstral cycle becomes less regular and ovulation is often unpredictable. The changs in hormone level cause many women to experience mood swings and physical symptoms, including cramps and hot flashes. This period when the ovaries stop producing viable secondary oocytes and the body becomes nonreproductive is known as the menopause. Occasionally the physical impairment become so severe that it interferes with nomal life and the enjoyment of sexual activity, and a physician might recommend hormonal treatment to augment the the natural production of hormones. Normally the sexual enjoyment of a healthy woman continues during the time of menopause and for many years thereafter.
Human males do not experience a relatively abrupt change in their reproductive and sexual lives. Rather, their sexual desires tend to wane slowly as they are. They produce fewer sperm cells and less seminal fluid. Healthy individuals can experience a satifying sex life during aging. Human sexual behavior is quite variable. The same is true of older persons. The whole range of responses to sexual partners continues but generally in a diminished form. People who were very active sexually when young continue to be active, but are less active as they reach middle age. Those who were less active tend to decrease their sexual activity also. It is reasonable to state that one’s sexuality continues from before birth until death.

    Although the s of the nervous and endocrine systems can overlay and be interrelated, these two systems have quite different methods of action. The nervous system s very much like a computer. A message is sent along established pathways from a specific initiating point to a specific end point, and the transmission is very rapid. The endocrine system s ina manner analogous to a radio broadcast system. Radio stations send their signals in all directions, but only those radio receivers that are tuned to the correct frequency can receive the message. Messageer molecules are typically distributed throughout the body by the circulatory system, but only those cells that have the proper receptor sites can receive and respond to the molecules.

the structure of the nervous system

The basic unit of the nervous system is a specialized cell called neuron, or nerve cell. A typical neuron consists of a central body called the soma, or cell body, which contains the nucleus and several long, protoplasmic extension called nerve fibers. There are two kinds of fiber; axons, which carry information away from the cell body, and dendrites, which carry information toward the cell body. Most nerve cells have one axon and several dendrites.

   Neurons are arranged into two major systems. The central nervous system, which consists of the brain and spinal cord, is surrounded by the skull and the vertebrae of the spinal column. It receives input from sense organs, interprets information, and generates responses. The peripheral nervous system is located outside the skull and spinal column and consists of bundles of long axons and dendrites called nerves. There are two different sets of neurons in teh peripheral nervous system. Motor neurons carry messages from the central nervous system to the muscles and glands, and sensory neurons carry input from sense organs to the central nervous system. Motor neurons typically have one long axon that runs from the spinal cord to a muscle or gland; sensory neurons have long dendrites that carry input from the sense organs to the central nervous system.

The nature of the nerve impulse

    Because most nerve cells have long fibrous extensions, it is possible for imformation to be passed along the nerve cell from one end to the other. The message that travels along a neuron is known as a nerve impulse. A nerve impulse is not like an electric current but involve a specific sequency of chemical events involving activities at the cell membrane.

    Because all cell membranes are differentially permeable, it is difficult for some ions to pass through the membrane and the combination of ions inside the membrane is different from that on the outside. Cell membranes also contain proteins that actively transport specific ions from one side of the membrane to the other. Active transport involves the cell's use of adenosione triphosphate to move mateirals from one side of the cell membrane to the other. Because ATP is required this is an ability that cells lose when they die. One of the ions that is actively transported from cells is the sodium in. At the same time sodium ions are being transported out of cells, potassium ions are being transported into the normal resting cells. However, there are more sodium ions transported out than potassium ions transported in.
  
    Because a normal resting cell has more positively charged Na+ ions on the outside of the cell than on the inside, a small but measurable voltage exists across the membrane of the cell. (voltage is a measure of the electricial charge difference that exists between two points or objects). The voltage difference between the inside and outside of a cell membrane is about 70 millivolts. the two sides of the cell membrane are , therefore, polarized in the same sense that a battery is polarized, with a positive and negative pole. A resting neuron has its positive pole on the outside of the cell membrane and its negative pole on the inside of the membrane.

    When a cell is stimulated at a specific point on the cell membrane, the cell membrane changes it permeability and let sodium ions pass through it from the outside to the inside. The membrane is thus depolarized; it loses its difference in charge as sodium ions diffuse into cell from the outside. sodium ions diffuse into the cell because, initially, they are in greater concentration outside the cell than inside. When the membrane becomes more permeable, they are able  to diffuse into the cell, toward the area of lower concentration. The depolarization of one piont on the cell membrane causes the adjacent portion of the cell membrane to change its permeability as well, and it also depolarizes. Thus a wave of depolarization passes along the length of the neuron from one end to the other . The depolarization and passage of an impulse along any portion of the neuron is a momentary event. As soon as a section of the membrane has been depolarized, potassium ions diffuse out of the cell. This re-establishes the original polarized state and the membrane is said to be repolarized. subsequently, the continuous active transport of sodium ions out of the cell and potassium ions into the cell restores the original concentration of ions on both sides of the cell membrane. When the nerve impuls reaches the end of the axon, it stimulates the release of a molecule that stimulates depolarization of the next neuron in the chain.

    Activities at the synapse

    Between the fibers of adjacent neurons in a chain is a space called the synapse. Many chemical events occur in the synapse that are important in the of the nervous system. When a neuron is stimulated, an impulse passes along its length from one end to the  other. When the impulse reaches a synapse, a molecule called a neurotransmitter is released into the synapse from the axon. it diffuses across the synapse and binds to specific receptor sites on the dendrite of the next neuron. When enough neurotransmitter molecules have been bound to the second neuron, an impulse is initiated in it as well. Several kinds of neurotransmitters are produced by specific neurons. These include dopamine, epinephrine, acetylcholine, and several other molecules. The first neurotransmitter indentified was acetycholine. Acetylcholine molecules are manufactured in the soma and migrate down the axon where they are stored until needed.

    As long as a neurotransmitter is bound to its receptor it continues to stimulate the nerve cell. Thus if acetylcholine continues to occupy receptors, the neuron continues to be stimulated again and again. An enzyme called acetylcholinesterase destroys acetylcholine and prevents this from happening. (The breakdown products of the acetylcholine can be used to remanufacture new acetylcholine molecules.) The destruction of acetylcholine allows the second neuron in the chain to return to normal. Thus it will be ready to accept another burst of acetylcholine from the first neuron a short time later. Neurons must also constantly manufacture new acetylcholine molecules or they will exhaust their supply and be unable to conduct an impulse across the synapse.

    Certain drugs, such as curare and strychnine, interfere with activities  at the synapse. Curare blocks the synapse and causes paralysis, whereas strychnine causes nerve cells to be continually stimulated. Many of the modern insecticides are also nerve poisons and are therefore quite hazardous.

    Because of the way the synapse works, impulse can go in only one direction: only axons secrete acetylcholine, and only dendrites have receptors. This explains why there are sensory and motor neurons to carry messages to and from the central nervous system.

    The nervous system is organized in a fashion similar to a computer. information from various input devices is delivered to the central processing unit by way of wires. The information is interpreted in the central processing unit. eventually messages can be sent by way of cables to drive external machinery (muscle and glands). This concept allows us to understand how the s of various portion of the nervous system have been identified. It is possible to electrically stimulate specific portions of the nervous system or to damage certain parts of the nervous system in experimental animals and determine the s of different parts of the brain and other parts of the nervous system. For example, because peripheral nerves carry bundles of both sensory and motor fibers, damage to a nerve may result in both a lack of feeling because sensory messages cannot get through and an inability to move because the motor nerves are damaged.

The organization of the central nervous system

Major s of specific portions of the brain have been identified. Certain parts of the brain are involved in controlling fundamental s such as breathing and heart rate. Others are involved in generating emotions, whereas others decode sensory input, or coordinate motor activity. The human brain also has considerable capacity to store information and creat new responses to environmental stimuli.

    The brain consists of several regions each of which has specific s. The s of the brain can be roughly divided into three major levels: automatic activities, basic decision making and emotions, and thinking and reasoning. If we begin with the spinal cord and work our way forward we will proceed from the more fundamental, automatic activities of the brain to the more complex thinking portions of the  brain. The spinal cord is a collection of nerve fibers surrounded by the vertebrae that convey information to and from the brain. At the base of the brain where the spinal cord enters the skull is a portion of the brain known as the medulla oblongata. This region of the brain controls fundamental activities such as blood pressure, breathing, and heart rate. Most of the fibers of the spinal cord cross from one side of the body to the other in the medulla oblongata. this is why the left side of the brain affects the right side of the body.

    the cerabellum is a large bulge at the base of the brain that is connected to the medulla oblongata. the primary of the cerebellum is coordination of muscle activity. It receives information from sense organs such as the portions of the ear that involve balance, the eyes, and pressure sensors in muscles and tendons. This information is used to make adjustments to the strength and order of contraction of muscle necessary to move in a coordinated fashion.

   The pons is connected to the anterior end of the medulla oblongata. It also connects to the cerebellum and to higher levels of the brain. It is involved in controlling many sensory and motor s of the sense organs of the head and face.

   If we continue forward the pons is connected to a portion of the brain that froms two bulblike structures that altimately connect to the cerebrum. Although these portions of the brain still control many automatic activities, there are many activities that involve much more integration of information and some level of " decision making" occurs in this region. The primary regions are the thalamus and the hypothalamus. The thalamus relays information between the cerebrum and lower portions of the brain. It also provides some level of awareness in that it it determines pleasant and unpleasant stimuli and is involved in sleep and arousal. The hypothalamus is also involved in sleep and arousal and is important in emotions auch as anger, fear, pleasure, hunger, sexual response, and pain. Several other more automatic s are regulated in this region, such as body temperature, blood pressure, and water balance. The hypothalamus also is connected to the pituitary gland and influences the manufacture and release of its hormones. Figure 20.6 shows the relationship of the various more primitive parts of the brain.

    The cerebrum is the largest portion of the brain in humans. The two hemispheres of the cerebrum cover all other portions of the brain except the cerebellum. the cerebrum is the thinking part of the brain. The surface of the cerebrum has been extensively mapped so that we know the location of many s. Abilities such as memory, language, control of movement, interpretation of sensory input, and thought are associated with specific areas of the cerebrum. figure 20.7 shows a diagram of the cerebrum and the locations of specific s.

   The of the brain is not determined by structure alone. Many parts of the brain have specialized neurons that produce specific neurotransmitter molecules used only to stimulate specific sensitive cells that have the proper receptor sites. As we learn more about the ing of the brain, we are finding more kinds of specialized neurotransmitter molecules. Their discovery allows for the treatment of many types of mental and emotional diseases. Manipulating these neurotransmitter molecules can help correct inappropriate ing of the brain. However, one should not assure that we understand the brain. We are still at an early stage in our research to comprehend this organ that sets us apart from other animals.

Endocrine system

As mentioned previously, the endocrine system is basically a broadcasting system in which glands secrete messenger molecules, called hormones, that are distributed throughout the body by the circulatory system. However, each kind of hormone affects only certain cells. The specific cells that a particular hormone affects are often called target cells. The hormones target certain cells because the cells have specific receptor molecules on their surfaces to which specific hormones attach. The cells that receive the messages typically respond in one of three ways: 1. some cells release products that have been previously manufactured, 2. other cells are stimulated to synthesize molecules or to begin bolic activities, and 3. some are stimulated to divide and grow.

   These different kinds of responses mean that some endocrine responses are relatively rapid, whereas others are very slow. For example, the release of the hormones epinephrine and norepinephrine from the adrenal medulla, located near the kidney, causes a rapid change in the behavior of an organism. The heart rate increases, blood pressure rises, blood is shunted to muscles, and the breathing rate increases. You have certainly experienced this reaction many times in your lifetime, such as when you nearly had an automobile accident or slipped and nearly fall.

   Another hormone, called antidiuretic hormone, acts more slowly. it is released from the posterior pituitary gland at the base of the brain and regulates the rate at which the body loses water through the kidneys. It does this by encouraging the reabsorption of water from their collecting ducts. the effects of this hormone can be noticed in a matter of minutes to hours. Insulin is another hormone whose effects are quite rapid. Insulin is produced by the pancreas, located near the stomach, and stimulates cells --particularly muscle, liver, and fat cells--to take up glucose from the blood. After a high carbohydrate meal, the level of glucose in the blood begins to rise, stimulating the pancreas to release insulin. The increased insulin causes glucose levels to fall as the sugar is taken up by cells. People with diabetes have insufficient or improperly acting insulin or lack the receptors to respond to the insulin, and therefore have difficulty regulating glucose levels in their blood.

   The response that result from the growth of cells may take weeks or years to occur. For example, growth stimulating hormone is produced by the anterior pitutitary gland over a period of years and results in typical human growth. After sexual maturity, the amount of this hormone generally drops to very low levels, and body growth stops. Sexual development is also largely the result of the growth of specific tissues and organs. The male sex hormone testosterone, produced by the testes, causes the growth of male sex organs and a change to the adult body form. The female counterpart, estrogen, results in the development of female sex organs and body form. In all of these cases, it is the release of hormones over long periods, continually stimulating the growth of sensitive tissues, that results in a normal developmental pattern. The absence or inhibition of any of these hormones early in life changes the normal growth process.
 
    Glands within the endocrine system typically interact with one another and control production of hormones. One commone control mechanism is called negative-feedback control. In negative-feedback control the increased amount of one hormone interferes with the production of a different hormone in the chain of events. The production of thyroxine and triiodothyronine by the thyroid gland exemflifies this kind of control. The production of these two hormone is stimulated by increased production of a hormone from the anterior pituitary called thyroid-stimualting-hormone. The control lies in the quantity of the hormone produced. When the anterior pituitary produces high levels of thyroid stimulating hormone, the thyroid is stimulated to grow and secrete more thyroxine and triiodothyronine. But when increased amounts of thyroxine and triiodothyronine are produced, these hormones ave a negative effect on the pituitary so that it decreases its production of thyroid-stimulating hormone, leading to reduced production of thyroxine and triiodothyronine. If the amount of the thyroid hormones falls too low, the pituitary is no longer inhibited and releases additional thyroid-stimulating hormone. As a result of the interaction of these hormones, their concentrations are maintaind within certain limits.

     It is possible for the nervous and endocrine systems to interact. The pituitary gland is located at the base of the brain and is divided into two parts. The posterior pituitary is directly connected to the brain and develops from nerve tissue. The other part, the anterior pituitary, is produced from the lining of the roof of the mouth in early fetal development. Certain pituitary hormones are produced in the brain and transported down axons to the posterior pituitary where they are stored before being released. The anterior pituitary also receives a continous input of messenger molecules from the brain, but these are delivered by way of a special set of blood vessels that pick up hormones produced by the hypothalamus of the brain and deliver them to the anterior pituitary.

     The pituitary gland produces a variety of hormones that are responsible for causing other endocrine glands, such as the thyoid, ovaries, and testes, and adrenals, to secrete their hormones. Pituitary hormones also influence milk production, skin pigmentation, body growth, mineral regulation, and blood glucose levels.

     Because the pituitary is constantly receiving information from the hypothalamus of the brain, many kinds of sensory stimuli to the body can affect the ing of the endocrine system. One example is the way in which the nervous system and endocrine system interact to influence the menstrual cycle. At least three different hormones are involved in the cycle of changes that affect the ovary and the lining of the uterus. It is well documented that stress caused by tension or worry can interfere with the normal cycle of hormones and delay or stop menstrual cycles. In addition, young women living in groups, such as in college dormitories, often find that their menstrual cycles become synchronized. Athough the exact mechanism involved in this phenomenon is unknown, it is suspected that imput from the nervous system causes this synchronization. (ordors and sympathetic feelings have been suggested as causes.)

     In many animals, the changing length of the day causes hormonal changes related to reproduction. In the spring, birds respond to lengthening days and begin to produce hormones that gear up their reproductive systems for the summer breeding season. The pineal body, a portion of the brain, serves as the receivers of light stimuli and changes the amounts of hormones secreted by the pituitary, resulting in changes in the levels of reproductive hormones. These hormonal changes modify the behavior of birds. courtship, mating, and nest-building behaviors increase in intensity. Therefore, it appears that a change in hormones level is affecting the behavior of the animal; the endocrine system is influenced the nervous system.

   It has been known for centuries that changes in  the levels of sex hormone cause changes in the behavior of animals. Castration (remove of the testes) of male domesticated animals, such as cattle, horses, and pigs, is sometimes done in part to reduce their aggressive behavior and make them easier to control. In humans, the use of anabolic steroids to increase muscle mass is known to cause behavioral changes and "moodiness".

  Although we still tend to think of the nervous and endocrine systems as being separate and different, it is becoming clear that they are interconnected. As we learn more about the molecules produced in the brain, it is becoming clear that the brain produces many molecules that act as hormones. some of these molecules affect adjacent parts of the brain, others affect the pituitary, and still others may have effects on more distant organs. In any case, these are two systems cooperate to bring about appropriate response to environmental challenges.The nervous system is specialized for receiving and sending short-term messages, whereas activities that require long-term, growth-related actions are handled by endorine system.

20.2 sensory input

The activities of the nervous and endocrine system are often response to some kind of input received from the sense organs. Sense organs of various types are located throughout the body. Many of them are located on the surface, where environmental changes can be easily detected. Hearing, sight, and touch are good examples of such senses. Other sense organs are located within the body and indicate to the organism how its various parts are changing. For example, pain and pressure are often used to monitor internal conditions. The sense organs detect changes, but the brain is responsible for perception--the recognition that a stimulus has been received. Sensory abilities involve many different kinds of mechanisms, including chemical recognition, the detection of energy changes, and the monitoring of physical forces.

   Chemical detection

All cells have receptors on their surfaces that can bind selectively to molecules they encounter. This binding process can cause changes in the cells in several ways. In some cells it causes depolarization. When this happens, the binding of molecules to the cell can stimulate neurons and cause messages to be sent to the central nervous system, informing it of some change in the surroundings. In other cases, a molecule binding to the cell surface may cause certain genes to be expressed, and the cell responds by changing the molecules it produces. This is typical of the way the endocrine system receives and delivers messages.

   Most cells have specific binding sites for particular molecules. others, such as the taste buds on the tongue, appear to respond to classes of molecules. Traditionally we have distinguished four kinds of tastes: sweet, sour, salt, and bitter. However, recently, a fifth kind of taste, umami (meaty), has been identified that responds to the amino acid, glutamate, which is present in many kinds of foods and is added as a flavor enhancer (monosodium glutamate) to many kinds of foods.
 
   The taste buds that give us the sour sensation respond to the presence of hydrogen ions. (acid foods taste sour). the hydrogen ions stimulate the cells in two ways: they enter the cell directly or they alter the normal movement of sodium and potassium ions across the cell membrane. In either case, the cell depolarizes and stimulates a nerve cell. sodium chloride stimulates the taste buds that give us the sensation of a salty taste by directly entering the cell, which causes the cell to polarize.

   However, the sensations of sweetness, bitterness, and umami occur when molecules bind to specific surface receptors on the cell. Sweetness can be stimulated by many kinds of organic molecules, including sugars and artificial sweeters, and also by inorganic lead compounds. When a molecule binds to a sweetness receptor, a molecule is split and its splitting stimulates an enzyme that leads to the depolarization of the cell. The sweet taste of lead salts in old paints partly explains why children sometimes eat paint chips. Because the lead interferes with normal brain development, this behavior can have disastrous results. Many other kinds of compounds of diverse structures give the bitter sensation. The cells that respond to bitter sensations have a variety of receptor molecules on their surface. When a substance binds to one of the receptors, the cell depolarizes. In the case of umami, It is the glutamate molecule that binds to receptors on the cells of the taste buds.

   Each of these tastes has a significance from a evolutionary piont of view. Carbohydrate are a major food source and many carbohydrates tastes sweet, therefore, this sense would be useful in identifying foods that have high food . Similarly, proteins and salts are necessary in the diet. Therefore, being able to indentify these itemss in potential foods would be extremely valuable. This is particularly ture for salt, which must often be obtained from meniral sources. On the other hand, bitter and sour materials are often harmful. Many plants produce toxic materials that are bitter tasting and acids are often the result of bacterial decomposition (spoiling) of foods. being able to identify bitter and sour would allow organisms to avoid foods that would be harmful.

   It is also important to understand that much of what we often refer to as taste involves such things as temperature, texture, and smell. Cold coffee has a different  taste than hot coffee even though they are chemically the same. Lumpy, cooked cereal and smooth cereal have different tastes. If you are unable to smell food, it doesn't taste as it should, which is why you sometimes lose your appetite when you have a stuffy nose. We still have much to learn about how the tongue detects chemicals and the role other associated senses play in modifying taste.

  The other major chemcial sense, the sense of smell, is much more versatile; it can detect thousands of different molecules at very low concentrations. The cells that makeup the olfactory epithelium, the cells that line the nasal cavtiy and respond to smells, apparently bind molecules to receptors on their surfaces. Exactly how this can account for the large number of recognizably different odors is unknown, but the receptor cells are extremely sensitive. In some cases a single molecule of a substance is sufficient to cause a receptor cell to send a message to the brain, where the sensation of odor is perceived. These sensory cells also fatigue rapidly. you have probably noticed that when you first walk into a room, specific odors are readily detected, but after a few minutes you are unable to detect them. Most perfumes and aftershaves are undetectable after 15 minutes of continuous stimulation.

   Many internal sense organs also respond to specific molecules. for example, the brain and aorta contain cells that respond to concentrations of hydrogen ions, carbon dioxide, and oxygen in the blood. Remember, too, that the endocrine system relies on the detection of specific messenger molecules to trigger its activities.

   Light detection

The eyes primarily respond to changes in the flow of light energy. The structure of the eye is designed to focus light on a light-sensitive layer of the back of the eye known as the retina of the eye. The cells called rods respond to a broad range of wavelengths of light and are responsible for black-and-white vision. Because rods are very sensitive to light, they are particularly useful in dim light. Rods are located over most of the retina surface except for the area of most acute vision known as the fovea centralis. The other receptor cells, called cones, are found throughout the retina but are particularly concentrated in the fovea centralis. Cones are not as sensitive to light. This combination of receptors gives us the ability to detect color when light levels are high, but we rely on black-and-white vision at night. There are three types of cones: one type responds best to red light, another responds best to green light, and the third responds best to blue light. Stimulation of various combinations of these three kinds of cones allows us to detect different shades of color.

   Rods and the three different kinds of cones each contain a pigment that decomposes when struck by light of the proper wavelength and sufficient strength. The pigment found in rods is called rhodopsin. This change in the structure of rhodopsin causes the rod to polarize. Cone cells have a similar mechanism of action, and each of the three kinds of cones has a different pigment. Because rods and cones synapse with neurons, they stimulate a neuron when depolarized and cause a message to be sent to the brain. Thus the pattern of color and light intensity recorded on the retina is detected by rods and cones and converted into a series of nerve impulses that are received and interpreted by the brain.

Sound detection

The ears respond to changes in sound waves. Sound is produced by the vibration of molecules. Consequently, the ears are detecting changes in the quantity of energy and the quality of the sound wave. Sound has several characteristics . Loudness, or volume, is a measure of the intensity of sound energy that arrives at the ear. Very loud sounds will literally wibrate your body, and can cause hearing lose if they are too intense. Pitch is a quality of sound that is determined by the frequency of the sound vibrations. High-pitched sounds have short wavelengths; low-pitched sounds have long wavelengths.

   Figure 20.14 shows the anatomy of the ear. The sound that arrives at the ear is first funneled by the external ear to the tympanum, also known as the eardrum. The cone-shaped nature of the external ear focuses sound on the tympanum and causes it to vibrate at the same frequency as the sound waves reaching it. Attached to the tympanum are three tiny bones known as the malleus (hammer), incus (anvil), and stapes (stirrup). The malleus is attached to the tympanum, the incus is attached to the malleus and stapes, and the stapes is attached to a small, membrane-covered opening called the oval window in a snail-shaped structure known as the cochlea. The vibration of the tympanum causes the tiny bones to vibrate, and they in turn cause a corresponding vibration in the membrane of the oval

   The cochlea of the ear is the structure that detects sound and consists of a snail-shaped set of fluid-filled tubes. When the oval window vibrates, the fluid in the cochlea begins to move, causing a membrane in the cochlea, called the basilar membrane, to vibrate. High-pitched , short-wavelength sounds cause the basilar membrane to vibrate at the base of the cochlea near the oval Low-pitched, long-wavelength sounds vibrate the basilar membrane far from the oval Loud sounds cause the basilar membrane to vibrate more vigoriously than do faint sounds. Cells on this membrane depolarize when they are stimulated by its vibrations. Because they synapse with neurons, messages can be sent to the brain.

   Because sounds of different wavelengths stimulate different portions of the cochlea, the brain is able to determine the pitch of a sound. Most sounds consists of a mixture of pitches that are heard. Louder sounds stimulate the membrane more forcefully, causing the sensory cells in the cochlea to send more nerve impulses per second. Thus the brain is able to perceive the loudness of various sounds as well as the pitch.

   Associated with the cochlea are two fluid-filled chambers and a set of fluid-filled tubes called the semicircular canals. These structures are not involved in hearing but are involved in maintaining balance and posture. In the wall of these chambers and canals are cells similar to those found on the basilar membrane. These cells are stimulated by movements of the head and by the position of the head with respect to the force of gravity. The constantly changing position of the head results in sensory input that is important in maintaining balance.

   Touch

   What we normally call the sense of touch consists of vaviety of different kinds of input. Some receptors respond to pressure, others to temperature, and others, which we call pain receptors, usually respond to cell damage. When these receptors are appropriately stimulated, they send a message to the brain. Because receptors are stimulated in particular parts of the body, the brain is able to localize the sensation. However, not all parts of the body are equally supplied with these receptors. The tips of the fingers, lips, and external genitals have the highest density of these nerve endings, whereas the back, legs, and arms have far fewer receptors.

  Some receptors , such as pain and pressure receptors, are important in allowing us to monitor our internal activities. many pains generated by the internal organs are often perceived as if they were somewhere else. For example, the pain associated with heart attack is often perceived to be in the left arm. Pressure receptors in joints and muscles are important to providing information about the degree of stress being placed on a portion of the body. this is also important information to send back to the brain so that adjustments can be made in movements to maintain posture. If you have ever had your foot "go to sleep" because the nerve stopped ing, you have experienced what is like to lost this constant input of nerve messages from the pressure sensors that assist in guiding  the movements you make. your movements become uncoordinated until the nerve returns to normal.

20.3 output coordination

The nerve system and endocrine system cause changes in several ways. Both systems can stimulate muscles to contract and glands to secrete. The endocrine system is also able to change the bolism of cells and regulate the growth of tissues. The nervous system acts upon two kinds of organs muscles and glands. the actions of muscles and glands are simple and direct: muscles contract and glands recrete.

Muscles

The ability to move is one of the fundamental characteristics of animals. Through the coordinated contraction of many muscles, the intricate, precise movements of a dancer, basketball plaer, or writer are accomplished. It is important to recognize that muscles can pull only by contracting; they are unable to push by lengthening. The work of any muscle is done during its contraction. Relaxation is the passive state of the muscle. There must always be some force available that will stretch a muscle after it has stopped contracting and relaxes. Therefore, the muscles that control the movements of the skeleton are present in antagonistic sets--foy every muscle's action there is another muscle that has the opposite action. For example, the biceps muscle causes the arm to flex as the muscle shortens. The contraction of its antagonist, the triceps muscle, causes the arm to extend (straighten) and at the same time stretchs the relaxed  biceps muscle.

    what we recognize as a muscle is composed of many muscle cells, which are in turn make up of myofibrils that are composed of two kinds of myofilaments. The mechanism by which muscle contracts is well understood and involves the movement of protein filaments past one another as ATP is utilized. ATP (adnosine triphosphate) is the primary molecule used by cells for their immediate energy needs. The filements in muscle cells are of two types, arranged in a particular pattern. Thin filements composed of the proteins actin, tropomyosin, and troponin alternate with thick filaments composed primarily of protein known as myosin.

   The myosin molecules have a shape similar to a golf club. the head of the club-shaped molecule sticks out from the thich filament and can combine with the action of the thin filament. However, the troponin and tropomyosin protein associated with the action cover the actin in such a way that myosin cannot bind with it. when actin is uncovered, myosin can bind to it and contraction of a muscle will occur when ATP is utilized.

  The process of muscle-cell contraction involves several steps. When a nerve impulse arrives at a muscle cell, its cells depolarize, calcium ions contained within membranes are released among the action and myosin filaments. The calsium ions combine with the tropnin molecules, causing the tropsin-tropomyosin complex to expose actin so that it can bind with myosin. While the actin and myosin molecules are attached, the head of the myosin molecule can flex as ATP is used and the actin molecule is pulled past the myosin molecule. Thus a tiny section of the muscle cell shortens. When one of our muscles contracts, thousands of such interactions take place within a tiny portion of a muscle cell, and many cells within a muscle all contract at the same time.

   There are three major types of muscle: skeletal, smooth, and cardiac. these differ from another in several ways. Skeletal muscle is voluntary muscle; it is under the control of the nervous system. The brain and spinal cord sends a message to skeletal muscles, and they contract to move the legs, fingers, and other parts of the body. This does not mean that you must make a consious decision every time you want to move a muscle. Many of the movements we make are learned initially but become automatic as a result of practice. for example, walking, swimming, or riding a bicycle required a great amount of practice originally, But now you probably perform these movements without thinking about them. They are,however, still considered voluntary actions.

  Skeletal muscles are constantly bombarded with nerve impulses that result in repeated contractions of different strength. Many neurons end in each muscle, and each one stimulates a specific set of muscle sells called a motor unit. Because each muscle consists of many motor units, it is possible to have a wide variety of intensities of contration within one muscle organ. This allows a single set of muscles to serve a wide variety of s. For example, the same muscles of the arms and shoulders that are used to play a piano can be used in other combinations to tightly grip and throw a baseball. If the nerves going to a muslce are destroyed, the muscle becomes paralyzed and begins to shrink. Regular nervous stimulation of skeletal muscle is necessary for muscle to maintain size and strength.any kind of prolonged inactivity leads to the degeneration of muscles known as astrophy. Muscle maintenance is one of the primary s of physical therapy and a benefit of regular exercise.

    Skeletal muscles are able to contract quickly, but they cannot remain contracted for long periods. Even when we contract a muscle for a minute or so, the muscle is constantly shifting the individual motor units within it that are in a state of contraction. A single skeletal muscle cell cannot stay in a contracted state.

   Smooth muscle make up the walls of muscular internal organs, such as the gut, blood vessels, and reproductive organs. They have the property of contracting as a response to being stretched. Because much of the digestive system is being stretched constantly, the responsive contractions contribute to the normal rhythmic movements associated with the digestive system. These are involuntary muscles; they can contract on their own without receiving direct messages from the nervous system. This can be demonstrated by removing portions of the gut or uterus from experimental animals. When these muscular organs are kept moist with special solution, they go through cycles of contraction without any possible stimulation from neurons. However, they do receive nervous stimulation, which can modify the rate and strength of their contraction. This kind of muscle also has the ability to stay contracted for long periods without fatigue. Many kinds of smooth muscle, such as the muscle of the uterus, also respond to the presence of hormones. Specially , the hormone oxytocin, which is released from the posterior pituitary, causes strong contractions of the uterus during labor and birth. similarly, several hormones produced by the duodenum influence certain muscles of the digestive system to either contract or relax.

   Cardiac muscle is the muscle that makes up the heart. It has the ability to contract rapidly like skeletal muscle, but does not require nervous stimulation to do so. Nervous stimulation can , however, cause the heart to speed or slow its rate of contraction. Hormones, such as epinephrine and norepinephrine, also influence the heart by increasing its rate and strength of contraction. Cardiac muscle also has the characteristic of being unable to stay contracted. It will contract quickly but must have a short period of relaxation before it will be able to contract a second time. This makes sense in light of its continuous, rhythmic, pumping . Table 20.1 summarizes the differenes among skeletal, smooth, and cardiac muscles.

   Glands

The glands of the body are of two different kinds. Those that secrete into the bloodstream are called endocrine glands. We have already talked about several of these: the pituitary, thyroid,ovary, and testis are examples. the exocrine glands are those that secrete to the surface of the body or into one of the tubular organs of the body, such as the gut or reproductive tract. Examples are the salivary glands, intestinal mucus glands, and sweat glands. Some of these glands, such as salivary glands and sweat glands, are under nervous control. When stimulated by the nervous system, they secrete their contents.

The Russian physiologist Ivan Petrovich Pavlov showed that salivary glands were under the control of the nervous system when he trained dogs to salivate in response to hearing a bell. You may recall from chapter 17 that, initially, the animals were presented with food at the same time the bell was rung. Eventually they would salivate when the bell was rung even if food was not present. This demonstrated that saliva release was under the control of the central nervous system.

   Many other exocrine glands are under hormonal control. Many of the digestive enzymes of the stomach and intestine are secreted in response to local hormone produced in the gut. These are circulated through the blood to the digestive glands, which respond by secreting the appropriate digestive enzymes and other molecules.

Growth responses

the hormones produced by the endocrine system can have a variety of effects. As mentioned earlier, hormones can stimulate smooth muscle to contract and can influence the contraction of cardiac muscle as well. Many kinds of glands, both endocrine and exocrine, are caused to secrete as result of a hormonal stimulus. However, the endocrine system has one major effect that is not  equaled by the nervous system: Hormones regulate growth. Several examples of the many kinds of long-term growth changes that are caused by the endocrine system were given earlier in the chapter. Growth-stimulating hormone is produced over a period of years to bring about the increase in size of most of the structures of the body. A low level of this hormone results in a person with small body size. It is important to recognize that the amount of growth-stimulating hormone present varies from time to time. It is present in fairly high amounts throughout childhood and results in steady growth. It also appears to be present at higher levels at certain times, resulting in growth spurts. Finally, as adulthood is reached, the level of this hormone falls, and growth stops.

   Similarly, testosterone produced during adolescence influences the growth of bone and muscle to provide men with larger, more muscular bodies than those of women. In addition, there is growth of penis, growth of the larynx, and increased growth of hair on the face and body. The primary female hormone, estrogen, causes growth of reproductive organs and development of breast tissue. It is also ivolves, along with other hormones in the cyclic growth and sloughing of the wall of the uterus.

Organisms maintain themselves by constantly processing molecules to provide building blocks for new living material and energy to sustain themselves. Autotrophs can manufacture organic molecules from inorganic molecules, but heterotrophs must consume organic molecules to get what they need. All molecules required to support living things are called nutrients. Some nutrients are inorganic molecules such as calcium, iron, or potassium; others are organic molecules such as carbohydrates, proteins, fats, and vitamins. All heterotrophs obtain the nutrients they need from food and each kind of heterotroph has particular nutritional requirements. This chapter deals with the nutritional requirements of humans.

The word nutrition is used in two related contexts. First, nutrition is a branch of science that seek to understand food, its nutrients, how the nutrients are used by the body, and how inappropriate combinations or quantities of nutrients lead to ill health. The word nutrition is also used in a slightly different context to refer to all the processes by which we take in food and utilize it, including ingestion, digestion, obsorption, and assimilation. Ingestion involves the process of taking food into the body through eating. Digestion involves the breakdown of complex food molecules to simpler molecules. Absorption involves the movement of simple molecules from the digestive system to the circulatory system for dispersal throughout the body. Assimilation involves the modification and incorporation of absorbed molecules into the structure of the organism.

Many of the nutrients that enter living cells undergo chemical changes before they are incorporated into the body. These interconversion processes are ultimately under the control of the genetic material, DNA. It is DNA that codes the information necessary to manufacture the enzymes required to extract energy from chemical bonds and to convert raw materials into the structure of the organism.

The food and drink consumed from day to day constitute a person’s diet. It must contain the minimal nutrients necessary to manufacture and maintain the body’s structure (bones, skin, tendon, musle, etc.) and regulatory molecules (enzymes and hormones), and to supply the energy (ATP) needed to run the body’s machinery. If the diet is deficient in nutrients, or if a person’s body cannot process nutrients efficiently,a dietary deficiency and ill health may result. A good understanding of nutrition can promote good health and involves an understanding of the nergy and nutrient content in various foods.

19.2 kilocalories, basal bolism, and weight control

The unit used to measure the amount of energy in foods is the kilocalorie. The amount of energy needed to raise the temperature of 1 kilogram of water 1 C is 1 kilocalorie. Remember the the prefix kilo- means “1000 times” the listed. Therefore, a kilocalorie is 1,000 times more heat energy than a calorie, which is the amount of heat energy needed to raise the temperature of 1 gram of water 1 C. however, the amount of energy contained in food is usually called a Calorie with a capital C. this is unfortunate because it is easy to confuse a Calorie, which is really a kilocalorie, with a calorie. Most books on nutrition and dieting use the term Calorie to refer to food calories. The energy requirements in kilocalories for a variety of activities are listed in table 19.1

Significant energy expeniture is required for muscular activity. However, even at rest, energy is required to maintain breathing, heart rate, and other normal body s. The rate at which the body uses energy when at rest is known as the basal bolic rate. The basal bolism of most people requires more energy than their voluntary muscular activity. Much of this energy is used to keep the body temperature constant. A true measurement of basal bolic rate requires a measurement of oxygen used over a specific period under controlled conditions. There are several factors that affect an individual’s basal bolic rate. Children have higher basal bolic rates and the rate declines throughout life. Elderly people have the lowest basal bolic rate. In general, males have higher bolic rates than women. Height and weight are also important. The larger a person the higher their bolic rate. With all of these factors taken into account, most young adults would fall into the range of 1,200 to 2,200 kilocalories for a basal bolic rate. Some other factors are: climate, altitude, physical condition, hormones, and previous diet, percent of weight that is fat, and time of the year.

Because few of us rest 24 hours a day, we normally require more than the energy needed for basal bolism. One of these requirements is the amount of energy needed to process food we eat. This is called specific dynamic action and is equal to approximately 10% of your total daily kilocalorie intake.

In addition to basal bolism and specific dynamic action, the activity level of a person determines the number of kilocalories needed. A good general indicator of the number of kilocalories neede above basal bolism is the type of occupation a person has. Since most adults are relatively sedentary, they would receive adequate amounts of women consumed 2,200 kilocalories and men consumed 2,900 kilocalories per day. Since approximately 60% of American are overweight or obese, the U.S. Department of Agriculture has developed a program aimed at educating people about the health hazards of obesity. One of the problems associated with obesity is identification—developing a good definition that can be easily understood. Table 19.3 shows guidelines for determining whether you are overweight or not. It is based on a specific method for determining body mass index—appropriate body weight compared to height. Body mass index is calculated by  determining a person’s weight (without clothing) in kilograms and barefoot height in meters. The body mass index is their weight in kilograms divided by their height in meters squared.

(the inside back over of this book gives conversions to the metric system of measurements.) for example, a person with a height of 5 feet 6 inches who weights 165 pounds has a body mass index of 26.6 kg/m2

Table 19.3 provides an easier way to determine your body mass index. Determine your weight withouth clothing and your height without shoes. Then go to table 19.3 to determine your body mass index.

The idea body mass index for maintaining good health is between 18.5 and 25 kg/m2. therefore, the person described above would be slightly over the recommended weight. Those with a body mass index between 25 and 30 kg/m2 are considered overweight, but there are no clear indications that there are significant health affects associated with this degree of overweight. Those with a body mass index of 30 kg/m2 or more hava a significant increased risk of many different kinds of diseases. The higher the body mass index the more significant the risk.

Why is weight control a problem for such a large portion of the population? There are several bolic pathways that convert carbohydrates or  proteins to fat. Stored body fat was very important for our prehistoric ancestors because it allowed them to survive periods of food scarcity. In periods of food scarcity the stored body fat can be used to supply energy. The glycerol portion of the fat can be converted to a small amount of glucose which can supply energy for red blood cells and nervous tissue that must have glucose. The fatty acid portion of the molecules can be bolized by most other tissues directly to produce ATP. Today, however, for most of us food scarcity is not a problem, and even small amounts of excess food consumed daily tend to add to our fat stores.

Although energy doesn’t weigh anything, the nutrients that contain the energy do. Weight control isa matter of balancing the kilocalories ingested as a result of dietary intake with the kilocalories of energy expended by normal daily activities and exercise. There is a limit to the rate at which a moderately active human body can use fat as an energy source. At most, 1 or 2 pounds of fat tissue per week are lost by an average person when dieting. Because 1 pound of fatty tissue contains about 3,500 kilocalories, decreasing your kilocalorie intake by 500 to 1,000 kilocalories per day while maintaining a balanced diet ( including proteins, carbohydrates,and fats) will result in fat loss of 1 or 2 pounds per week. ( a pound of pure fat contains about 4,100 kilocalories, but fat tissue contains other materials besides fat, such as water.)

Many diets promise large and rapid weight loss but in fact result only temporary water loss. They may encourage eating and drinking foods that are diuretics, which increase the amount of urine produced and thus increase water loss. Or they many encourage exercise or other activities that cause people to lose water through sweating. Low carbohydrate diets deprive the body of glucose needed to sustain nervous tissue and red blood cells. If glucose is not available the body will begin to use protein from the liver and muscles to provide the glucose needed for these vital cells. This kind of weight loss is not healthy. Finally, just reducing the amount of food in the gut by fasting resulting in a temporary weight loss because the gut is empty.

For those who need to gain weight, increasing kilocalorie intake by 500 to 1,000 kilocalories per day will result in an increase of 1 or 2 pounds per week, provided the low weight is not the result of health problem.

If you have calculated your body mass index and wish to modify your body weight, what are the steps you should take? First, you should chech with your physician before making any drastic change in your eating habits. Second, you need to determine the number of kilocalories you are consuming. That means keeping an accurate diet record for at least a week. Record everything you eat and drink and determine the number of kilocalories in those nutrients. This can be done by estimating the amounts of protein, fat, and carbohydrate (including alcohol) in your foods. Roughly speaking, 1 gram of carbohydrate is the equivalent of 4 kilocalories, 1 gram of fat is the equivalent of 9 kilocalories, 1 gram of protein is the equivalent of 4 kilocalories, and 1 gram of alcohol is 7 kilocalories. Most nutrition books have food-composition tables that tell you how much protein, fat, and carbohydrate are in a particular food. Packaged foods also have serving sizes and nutritve content printed on the package. Do the arithmetic and determine your total kilocalorie intake for the week. If your intake in kilocalories equals your output, you should not gain any weight! You can double-check this by weighing yourself before and after your week of record keeping. If your weight is constant and you want to lose weight, reduce the amount of food in your diet. To lose 1 pound each week, reduce your kilocalorie intake by 500 kilocalories per day. Be careful not too eat less than 600 kilocalories of carbohydrates or reduce total daily intake below 1,200 kilocalories unless you are under the care of a physician. It is important to have some carbohydrate in your diet because a lack of carbohydrate leads to a breakdown of the protein that provides the cells with the energy they need. Also you may not be getting all the vitamins required for efficient bolism and you could cause yourself harm. To gain 1 pound, increase your intake by 500 kilocalories per day.

A second ingredient valuable in a weight loss plan is an increase in exercise while keeping food intake constant. This can involve organized exercising in sports or fitness programs. It can also include simple things like walking up the stairs rather than taking the elevator, parking at the back of the parking lot so that  you walk farther, riding a bike for short errands, or walking down the hall to someone’s officer rather than using the phone. Many people who initiate exercise plans as a way of reducing weight are frustrated because they may initially gain weight rather than lose it. This is because muscle weights more than fat. Typically they are “out of shape” and have low muscle mass. If they gain a pound of muscle at the same time they lose a pound of fat they will not lose weight. However, if the fitness program continues they will eventually reach a point where they are not increasing muscle mass and weight loss will occur. Even so, weight as muscle is more healthy than weight as fat.

If, like millions of others, you believe that you are overweight, you have probably tried numerous diet plans. Not all of these plans are the same, and not all are suitable to your particular situation. If a diet plan is to be valuable in promoting good health, it must satisfy your needs in several ways. It must provide you with needed kilocalories, proteins, fats, and carbohydrates. It should also contain readily avaible foods from all the basic food groups, and it should provide enough variety to prevent you from becoming bored with the plan and going off the diet too soon. A diet should not be something you follow only for a while, then abandon and regain the lost weight.

Living things are complex machines with many parts that must work together in a coordinated fashion. All systems are integrated and affect one another in many ways. For example, when you run up a hill, your leg and arm muscles move in a coordinated way to provide power. They burn fuel (glucose) for energy and produce carbon dioxide and lactic acid as waste products, which tend to lower the pH of the blood. Your heart beats faster to provide oxygen and nutrients to the muscles, your breathe faster to supply the muscles with oxygen and to get rid of carbon dioxide, and the blood vessels in the muscles dilate to allow more blood to flow to them. As you run you generate excess heat. As a result, more blood flows to the skin to get rid of the heat and sweat glands begin to secret, thus cooling the skin. All of these automatic internal adjustments help the body maintain a constant level of oxygen, carbon dioxide, and glucose in the blood; constant pH; and constant body temperature. They can be summed up in the concept of homeostasis. Homeostasis is the maintenance of a constant internal environment as a result of monitoring and modifying the ing of various systems. To explore the various mechanisms that help organisms maintain homeostasis, we will begin at the cellular level.

Cells are highly organized units that require a constant flow of energy in order to maintain themselves. The energy they require is provided in the form of nutrient molecules that enter the cell. Oxygen is required for the efficient release of energy from the large organic molecules that serve as fuel. Inevitably, as oxidation takes place, waste products form that are useless or toxic. These must be removed from the cell. All these exchanges of food, oxygen, and waste products must take place throught the cell surface.

As a sell grows its volume increases, and the amount of bolic activity required to maintain ir rises. The quantity of materials that must be exchanged between the cell and its surroundings increases. Thus growth cannot continue indefinitely. The ultimate size of a cell is limited by one or more of the following interrelated factors:

1.                          the strength of the membrane: as the cell increases in size, the membrane will eventually not be strong enough to withstand the forces caused by the mass of material inside it. If you had a balloon and kept adding water to it, eventually the balloon would burst. Similarly, dams have failed when too much water was accumulated behind them.

2.                          the cell surface area: if materials are to enter a cell they must pass through a surface. The cell membrane is a selectively permeable barrier to the passage of material in and out of the cell. The amount of surface will determine how much material can pass. If you were to pour water through two coffee filters of different size, the one with the largest surface area would allow the water to pass through more rapidly.

3.                          the surface area-to-volume ratio: the bolic needs of a cell are determined by its volume and the ability of exchange materials between the cell and its surroundings are determined by its surface area. As a cell increases in size, its volume increases faster than its surface area. This relationship between surface area and volume is often expressed as the surface area-to-volume ratio.

The ability to transport materials into or out of a cell is determined by its surface area, whereas its bolic demands are determined by its volume. So the larger a cell becomes, the more difficult it is to satisfy its needs. Some cells overcome this handicap by having highly folded cell memebranes that substantially increase their surface areas. This is particularly true of cells that line the intestine or are involved in the transport of large numbers of nutrient molecules. These cells have many tiny, folded extensions of the cell membrane called microvilli.

In similar way, the structure of an automobile radiator increases the efficiency of heat exchange between the engine and the air. The radiator has many fins attached to tubes through which a coolant fluid is pumped. Because of the large surface area provided by the fins, heat from the engine can be efficiently radiated away.

In addition to the limitation that surface area presents to the transport of materials, large cells also have a problem with diffusion. The molecular process fo diffusion is quite rapid over short distances but becomes very slow over longer distances. Diffusion is generally insufficient to handle the needs of cells if it must take place over a distance of more than a millimeter. The center of the cell would die before it received the molecules it needed if the distance were greater. Because of this and the problems presented by the surface area-to-volume ratio, it is understandable that the basic unit of life, the cell, must remain small.

All single-celled organism are limited to a small body size because they handle the exchange of molecules through their cell membranes. Large, multicellular organisms consist of a multitude of cells, many of which are located far from the surface of the organism. Each cell within a multicellular organism must solve the same materials-exchange problems as single-celled organisms. Large organisms have several interrelated systems that are involved in the exchange and transport of materials so that each cell can meet its bolic needs. Diffusion, facilitated diffusion, and active transport are all involved in moving molecules across cell membranes. These  topics are presented in detail in chapter 4.

18.2 circulation

Large, muticellular organisms like humans consist of trillions of cells. Because many of these cells are buried within the organism far from the body surface, there must be some sort of distribution system to assist them in solving their materials-exchange problems. The primary mechanism used is the circulatory system.

    The circulatory system consists of several fundamental parts. Blood is the fluid medium that assists in the transport of materials and heat. The heart is a pump that forces the fluid blood from one part of the body to another. The heart pumps blood into arteries, which distribute blood to organs. It flows into successively smaller arteries until it reaches tiny vessels called capillaries, where materials are exchanged between the blood and tissues through the walls of the capillaries. The blood flows from the capillaries into veins that combine into larger veins that ultimately return the blood to the heart from the tissues.

The nature of blood

Blood is a fluid that consists of several kinds of cells suspended in a watery matrix called plasma. This fluid plasma also contains many kinds of dissolved molecules. The primary of the blood is to transport molecules, cells, and heat from one part of the body to another. The major kinds of molecules that are distributed by the blood are respiratory gases ( oxygen, and carbon dioxide), nutrients of various kinds, waster products, disease-fighting antibodies, and chemical messengers (hormones). Blood has special characteristics that allow it to distribute respiratory gases very efficiently. Although little oxygen is carried as free, dissolved oxygen in the plasma, red blood cells, contain hemoglobin, an iron-containing molecule, to which oxygen molecules readily bind. This allows for much more oxygen to be carried than could be possible if it were simply dissolved in the blood. Because hemoglobin is inside red blood cells, it is possible to assess certain kinds of health problems by counting the number of red blood cells. If the number is low, the person will not be able to carry oxygen efficiently and will tire easily. This condition, in which a person has reduced oxygen-carrying capacity, is called anamia. Anemia can also result when a person does not get enough iron. Because iron is a central atom in hemoglobin molecules, people with an iron deficiency are not able to manufacture sufficient hemoglobin. They can be anemic even though their number of red blood cells may be normal.

Red blood cells are also important in the transport of carbon dioxide. Carbon dioxide is produced as a result of normal aerobic respiration of food materials in the cells of the body. If it is not eliminated, it causes the blood to become more acidic (lowers its pH), eventually resulting in death. Carbon dioxide can be carried in the plasma, about 23% is carried attached to hemoglobin molecules, and 70% is carried as bicarbonate ions. An enzyme in red blood cells known as carbonic anhydrase assists in converting carbon dioxide into bicarbonate ions, which can be carried as dissolved ions in the plasma of the blood. The following reversible chemical equation shows the changes that occur.

When the blood reaches the lungs, dissolved carbon dioxide is lost from the palsma, and carbon dioxide is released from the hemoglobin molecules as well. In addition, the bicarbonate ions reenter the red blood cells and can be converted back into molecular carbon dioxide by the same enzyme-assisted process that converts carbon dioxide to bicarbonate ions. The importance of this mechanism will be discussed later when the exchange of gases at lung surface is described.

Heat is also transported by the blood. Heat is generated by bolic activities and must be lost from the body. To handle excess body heat, blood is shunted to the surface of the body, where heat can be radiated away. In addition, humans and some other animals have the ability to sweat. The evaporation of sweat from the body surface also gets rid of excess heat. If the body is losing heat to rapidly, blood flow is shunted away from the skin, and bolic heat is conserved. Vigorous exercise produces an excess of heat so that, even in cold weather, blood is shunted to the skin and skin feels hot.

The plasma also carries nutrient molecules from the gut to other locations where they are modified, bolized, or incorporated into cell structures. Amino acids and simple sugars are carried as dissolved molecules in the blood. Lipids, which are not water soluble, are combined with proteins and carried as suspended particles, called lipoproteins. Most lipids do not enter the bloodstream directly from the small intestine but are carried to the bloodstream by the lymphatic system. Other organs, like liver, manufacture or modify molecules for use elsewhere; therefore they must constantly receirve raw materials and distribute their products to the cells that need them through the transportation of the blood.

In addition, many different kinds of hormones are produced by the brain, reproductive organs, digestive organs, and glands of the body. These are secreted into the bloodstream and transported throughout the body. Tissues with appropriate receptors bind to these molecules and respond to these chemical messengers.

The immune system           

Table 18.1 lists tha variety of cells found in blood. Whereas the red, hemoglobin-containing erythrocytes serve in the transport of oxygen and carbon dioxide, the white blood cells carried in the blood are involved in defending against harmful agents. These cells help the body resist many diseases. They constitute the core of the immune system. The various WBCs participate in providing immunity in several ways. First, immune system cells are able to recognize cells and molecules that are foreign to the body. If a molecules is recognized as foreign, certain WBCs produce antibodies that attach to the foreign materials. The foreign molecules that stimulate the production of antibodies are called antigens. When harmful microorganisms (e.g., bacteria, viruses, fungi), cancer cells, or toxic molecules enter the body, other WBCs recognize, boost their abilities to respond to, move toward, and destroy the problem causers. Neutrophils, eosinophils, basophils, and monocytes are specific kinds of WBCs capable of engulfing foreign material, a process called phagocytosis. Thus they are often called phagocytes. Although most can move from the bloodstream into the surrounding tissue, monocytes undergo such a striking increase in size that they are given a different –macrophages. Macrophages can be found throughout the body and are the most active of the phagocytes.

   The other major type of white cells, lymphocytes, work with phagocytes to provide protection. The two major types are T-lymphocytes and B-lymphocytes. T-cells are involved in a cell-mediated immune response in which cells directly attack potentially dangerous objects. This highly complex response involves the release of chemical messengers that coordinate the response, an increase in the population of T-cells and B-cells, and stimulation of B-cell and macophage activities. Some T-cells are capable of killing dangerous cells by destroying their cell membranes. T-cells are primarily involed in fighting infections of viruses, fungi, protozoa, worms, and cancer cells.

B-cells are involved in antibody-mediated immunity in which B-cells produce antibody molecules that are released into the blood stream and are distributed to all parts of the body. These antibodies attach to the foreign molecules, causing them to clump together. This clumping may destroy their harmful properties, make them more susceptible to chemical attack, or make them more recognizable for phagocytes. Many kinds of disease caused by viruses and bacteria are controlled by antibodies produced by B-cells.

Another kind of cellular particle in the blood is the platelet. These are fragments of specific kinds of white blood cells and are important in blood clotting. They collect at the site of a wound where they break down, releasing molecules. This begins a series of reations that results in the formation of fibers that trap blood cells and form a plug in the opening of the wound.

The heart

Blood can perform its transportation only if it moves. The organ responsible for providing the energy to pump the blood is the heart. In order for a fluid to flow through a tube, there must be a pressure difference between the two ends of the tube. Water flows through pipes because it is under pressure. Because the pressure is higher behind a faucet than at the spout, water flows from the spout when the faucet is opened. The circulatory system can be analyzed from the same point of view. The heart is a muscular pump that provides the pressure necessary to propel the blood throughout the body. It must continue its cycle of contraction and relexation, or blood stops flowing and body cells are unable to obtain nutrients or get rid of wastes. Some cells, such as brain cells, are extremely sensitive to having their flow of blood interrupted because they require a constant supply of glucose and oxygen. Others, such as muscle cells or skin cells, are much better able to withstand temporary interruptions of blood flow.

The hearts of humans, other mammals, and birds consist of four chambers and four sets of valves that work together to ensure that blood flows in one direction only. Two of these chambers, the right and left atria (singular, atrium), are relatively thin-walledd structures that collect blood from the major veins and empty it into the larger, more muscular ventricles. Most of the flow of blood from the atria to the ventricles is caused by the lowered pressure produced within the ventricles as they relax. The contraction of the thin-walled atria assists in emptying them more completely.

The right and left ventricles are chambers that have powerful muscular walls whose contraction forces blood to flow through the arteries to all parts of the body. The valves between the atria and ventricles, known as atrioventricular valves, are important one-way valves that allow the blood to flow from the atria to the ventricles but prevent flow in the opposite direction. Similarly, there are valves in the aorta and pulmonary artery, known as semilunar valves, the aorta is the large artery that carries blood from the left ventricle to the body, and the pulmonary artery carries blood from  the right ventricle to the lungs. The semilunar valves prevent blood from flowing back into the ventricles. If the atrioventricular or semilunar valves are damaged of improperly, the efficiency of the heart as a pump is diminished, and the person may develop an enlarged heart or other symptoms. Maling heart valves are often diagnosed because they cause abnormal sounds as the blood passes through them. These sounds are referred to as heart murmurs. Similarly, if the muscular walls of the ventricles are weakened because of infection, damage from a heart attack, or lack of exercise, the pumping efficiency of the heart is reduced and the person develops symptoms that may include chest pain, shortness of breath, or fatigue. The pain is caused by an increase in the amount of lactic acid in the musle because the heart muscle is not getting sufficient blood to satisfy its needs. It si important to understand that the muscle of the heart receives blood from coronary arteries that are branches of tha aorta. It is not nourished by the blood that flows through its chambers. If heart muscle does not get sufficient oxygen for a peroid of time, the portion of the heart muscle not receiving blood dies. Shortness of breath and fatigue result because the heart is not able to pump blood efficiently to the lungs, muscles, and other parts of the body.

The right and left sides of the heart have slightly different jobs because they pump blood to different parts of the body. The right side of the heart receives blood from the general body and pumps it through the pulmonary arteries to the lungs, where exchange of oxygen and carbon dioxide takes place and the blood returns from the lungs to the left atrium. This is called pulmonary circulation. The larger, more powerful left side of the heart receives blood from the lungs, delivers ir through the aorta to all parts of the body, and returns it to the right atrium by way of veins. This is known as systemic circulation. Both circulatory pathway are shown in figure 18.4. the systemic circulation is responsible for gas, nutrient, and waste exchange in all parts of the body except the lungs.

Arteries and veins

Arteries and veins are the tubes that transport blood from one place to another within the body. Figure 18.5 compares the structure and of arteries and veins. Arteries carry blood away from the heart because it is under considerable pressure from the contraction of the ventricles. The contraction of the walls of the ventricles increases the pressure in the arteries. A typical pressure recorded in a large artery while the heart is constracting is about 120 millimeters of mercury. This is known as the systolic blood pressure. The pressure recorded while the heart is relaxing is about 80 millimeters of mercury. This is known as the diastolic blood pressure. A blood pressure reading includes both numbers and is recored as 120/80. (originally, blood pressure was measured by how high the pressure of the blood would cause a column of mercury to rise in a tube. Although the devices used today have dials or digital readouts and contain no mercury, they are still calibrated in mmHg.

The walls of arteries are relatively thick and muscular, yet elastic. Healthy arteries have the ability to expand as blood is pumped into them and return to normal as the pressure drops. This ability to expand absorbs some of the pressure and reduces the peak pressure within the arteries, thus reducing the likelihood that they will burst. If arteries become hardened and less resilient, the peak blood pressue rises and they are more likely to rupture. The elastic nature of the arteries is also responsible for assisting the flow of blood. When they return to normal from their stretched condition they give a little push to the blood that is flowing through them.

Blood is distributed from the large aorta through smaller and smaller blood vessels to millions of tiny capillaries. Some of the smaller arteries, called arterioles, may contract or relax to regulate the flow of blood to specific parts of the body. Major parts of the body that receive differing amounts of blood, depending on need, are the digestive system, muscles, and skin. When light-skinned people flush, it is because many arterioles in the skin have expanded, allowing a large volume of blood to flow to the capillaries of the skin. Because the blood is red, their skin reddens. Similarly, when people exercise, there is an increased blood flow to muscles to accommodate their increased bolic needs for oxygen and glucose and to get rid of wastes. Exercise also results in an increased flow of blood to the skin, which allows for heat loss. At the same time, the amount of blood flowing to the digestive system is reduced. Athletes do not eat a full meal before exercising because the additional flow of blood to the digestive system reduces the amount of blood available to go to muscles and lungs needed for vigorous exercise. Muscular cramps may results if insufficient blood is getting to the muscles.

Veins collect blood from the capillaries and return it to the heart. The pressure in these blood vessels is very low. Some of the largest veins may have a blood pressure of 0.0 mmHg for brief periods. The walls of veins are not as muscular as those of arteries. Because of the low pressure, veins must have valves that prevent the blood from flowing backward, away from the heart. Veins are often found at the surface of the body and are seen as blue lines. Varicose veins results when veins contain faulty valves that do not allow efficient return of blood to the heart. Therefore, blood pools in these veins, and they become swollen bluish networks.

Because pressure in veins is so low, muscular movements of the body are important in helping return blood to the heart. When muscles of the body contract, they compress nearby veins, and this pressure pushes blood along in the veins. Because the valves allow blood to flow only toward the heart, this activity acts as an additional pump to help return blood to the heart. People who sit or stand for long periods without using their muscles tend to have a considerable amount of blood pool in the veins of their legs and lower body. Thus less blood may be available to go to the brain and the person may faint.

Although the arteries are responsible for distributing blood to various parts of the body and arterioles regulate where blood goes, it is the of capillaries to assist in the exchange of materials between the blood and cells.

Capillaries

Capillaries are tiny, thin-walled tubes that receive blood from arterioles. They are so small that red blood cells must go through them in single file. They are so numerous that each cell in the body has a capillary located near it. It is estimated that there are about 1,000 square meters of surface area represented by the capillary surface in a typical human. Each capillary wall consists of a single layer of cells and therefore presents only a thin barrier to the diffusion of materials between the blood and cells. It is also possible for liquid to flow through tiny spaces between the individual cells of most capillaries. The flow of blood through these smallest blood vessels is relatively slow. This allows time for the diffusion of such materials as oxygen, glucose, and water from the blood to surrounding cells, and for the movement of such materials as carbon dioxide, lactic acid, and ammonia from the cells into the blood.

In addition to molecular exchange, considerable amounts of water and dissolved materials leak through the small holes in the capillaries. This liquid is known as lymph. Lymph is produced when the blood pressure forces water and some small dissolved molecules through the walls of the capillaries. Lymph bathes the cells but must eventually be returned to the circulatory system by lymph vessels or swelling will occur. Return is accomplished by the lymphatic system, a collection of thin-walled tubes that branch through out the body. These tubes collect lymph that is filtered from the circulatory system and ultimately empty it into major blood vessels near the heart. As the lymph makes its way back to the circulatory system, it is filtered by lymph nodes that contain large numbers of white blood cells that remove microorganisms and foreign particles. There are many lymph nodes located throughout the body. The tonsils and adnoids are large masses of lymph node tissue. The speen also contains large numbers of white blood cells and serves to filter the blood. The thymus gland is located over the breastbone and is large and active in children. Its primary is to produce T-lymphocytes that are distributed throughout the body and establish themselves in lymph nodes. The thymus shrinks in size in adulthood, but the descendants of the T-lymphcytes it produced earlier in life are still active throughout the lymphatic system. Figure 18.7 shows the structure of the lymphatic system.

Some of this leakage through the capillary walls is normal, but the flow is subject to changes in pressure inside the capillaries and in the tissues, and changes in the permeability of capillary wall. If pressure inside the capillary increases, more fluid may leak from the capillaries into the tissues and cause swelling. This swelling is called edema, and it is common in circulatory disorders. Another cause of edema is an increase in the permeability of the capillaries. This is commonly associated with injury to a part of the body: a sprained ankle or smashed thumb are examples you have probably experienced.

18.3 gas exchange

The lungs demonstrate this interplay between blood flow, caplillary exchange, and surface area.

Respiratory anatomy

The lungs are organs of the body that allow gas exchange to take place between the air and blood. Associated with the lungs is a set of tubes that conduct air from outside the body to the lungs. The single large-diameter trachea is supported by rings of cartilage that prevent its collapse. It branches into two major bronchi (singular, bronchus) that deliver air to smaller and smaller branches. Bronchi are also supported by cartilage. The smallest tubes, known as bronchioles, contain smooth muscle and are therefore capable of constricting. Finally, the bronchioles deliver air to cluster of tiny sacs, known as alveoli (singular, alveolus), where the exchange of gases takes place between the air and blood.

The nose, mouth, and throat are also important parts of the air-transport pathway because they modify the humidity and temperature of the air and clean the air as it passes. The lining of the trachea contains cells with cilia that beat in a direction to move mucus and foreign materials from the lungs. The foreign matter may then be expelled by swallowing, coughing, or other means. Figure 18.8 illustrates the various parts of the respiratory system.

Breathing system regulation

Breathing is the process of moving air in and out of the lungs. It is accomplished by the movement of a muscular organ known as diaphragm, which separates the chest cavity and the lungs from the abdominal cavity. In addition, muscles located between the ribs (intercostal muscles) are attached to the ribs in such a way that their contraction causes the chest wall to move outward and upward, which increases the size of the chest cavity. During inhalation, the diaghragm moves downward and the external intercostal muscles of the chest wall contract, causing the volume of the chest cavity to increase. This results in a lower pressure in the chest cavity compared to the outside air pressure. Consequently, air flows from the outside high-pressure area through the trcachea, bronchi, and bronchioles to the alveoli. During normal relaxed breathing, exhalation is accomplished by the chest wall and diaphragm simply returning to their normal, relaxed positions. Muscular contraction is not involved.

However, when the body’s demand for oxygen increases during exercise, the only way that the breathing system can respond is by exchanging the gases in the lungs more rapidly. This can be accomplished both by increasing the breathing rate and by increasing the volume of air exchangeed with each breath. Increase in volume exchanged per breath is accomplished in two ways. First, the muscles of inhalation can contract more forcefully, resulting in a greater change in the volume of the chest cavity. In addition, the lungs can be emptied more completely by contracting the muscles of the abdomen, which forces the abdominal contents upward against the diaphragm and compresses the lungs. A set of internal intercostal muscles also helps compress the chest. You are familiar with both mechanisms: when you exercise you breathe more deeply and more rapidly.

Several mechanisms can cause changes in the rate and depth of breathing, but the primary mechanism involves the amount of carbon dioxide present in the blood. Carbon dioxide is a waste product of aerobic cellular respiration and becomes toxic in high quantities because it combines with water to form carbonic acid:

As mentioned previously, if carbon dioxide can not be eliminated, the pH of the blood is lowered. Eventually, this may result in death.

Exercising causes an increase in the amount of carbon dioxide in the blood because muscles are oxidizing glucose more rapidly. This lowers the pH of the blood. Certain brain cells and specialized cells in aortic arch and carotid arteries are sensitive to changes in blood pH. When they sense a  lower blood pH, nerve impulses are sent more frequently to the diaphragm and intercostal muscles. These muscles contract more rapidly and more forcefully, resulting in more rapid, deeper breathing. Because more air is being exchanged per minute, carbon dioxide is lost from the lungs more repidly. When exercise stops, blood pH rises, and breathing eventually return to normal. Bear in mind, however, that moving air in and out of the lungs is of no unless oxygen is diffusion into the blood and carbon dioxide is diffusion out.

Lung

The lungs are organs that allow blood and air to come in close contact with each other. Air flows in and out of the lungs during breathing. The blood flows through capillaries in the lungs and is in close contact with the air in the cavities of the lungs. For oxygen to enter the body or carbon dioxide to exit the body the molecules must pass through a surface. The efficiency of exchange is limited by the surface area available. This problem is solved in the lungs by the large number of tiny sacs, the alveoli. Each alveolus is about 0.25 to 0.5 millimeters across. However, alveoli are so numerous that the total surface area of all these sacs is about 70 square meters—comparable to the floor space of many standard-sized classrooms! Associated with these alveoli are large numbers of capillaries. The walls of both capillaries and alveoli are very thin, and the close association of alveoli and capillaries in the lungs allows the easy diffusion of oxygen and carbon dioxide across these membranes.

Another factor that increases the efficiency of gas exchange is that both the blood and air are moving. Because blood is flowing through capillaries in the lungs, the capillaries continually receive new blood that is poor in oxygen and high in carbon dioxide. As blood passes by the alveoli, it is briefly exposed to the gases in the alveoli, where it gains oxygen and loses carbon dioxide. Thus, blood that leaves the lungs is high in oxygen and low in carbon dioxide. Although the movement of air in the lungs is not in one direction, as is the case with blood, the cycle of inhalation and exhalation allows air that is high in carbon dioxide and low in oxygen to exit the body and brings in new air that is rich in oxygen and low in carbon dioxide. This oxygen-rich blood is then sent to the left side of the heart and pumped throughout the body.

Any factor that interferes with the flow of blood or air or alters the effectiveness of gas exchange in the lungs reduces the efficiency of the organism. A poorly pumped heart sends less blood to the lungs, and the person experiences shortness of breath as a symptom. Similarly, diseases like asthma, which causes constriction of the bronchioles, reduce the flow of air into the lungs and inhibit gas exchange.

Any process that reduces the number of alveoli will also reduce the efficiency of gas exchange in the lungs. Emphysema is a progressive disease in which some of the alveoli are lost. As the disease progresses, those afflicted have less and less respiratory surface area and experience greater and greater difficulty getting adequate oxygen, even though they may be breathing more rapidly. Often emphysema is accompanied by an increase in the amount of connective tissue and the lungs do not streth as easily, further reducing the ability to exchange gases.

The breathing mechanism is designed to get oxygen into the bloodstream so that it can be distributed to the cells that are carrying on the oxidation of food molecules, such as glucose and fat. Obtaining food molecules involves a variety of organs and activities associated with the digestive system.

18.4 obtaining nutrients

All cells must have a continuous supply of nutrients that provides the energy they require and the building blocks needed to construct the macromolecules typical of living things. They specific functioons of various kinds of nutrients are discussed in chapter 19. this section will deal with the processing and distribution of different kinds of nutrients.

The digestive system consists of a muscular tube with several specialized segments. In addition, there are glands that secrete digestive juices into the tube. Four different kinds of activities are involved in getting nutrients to the cells that need them: mechanical processing, chemical processing, nutrient uptake, and chemical alteration.

Mechanical and chemical processing

The digestive system is designed as a disassembly system. Its purpose is to take large chunks of food and break them down to smaller molecules that can be taken up by the circulatory system and distributed to cells. The first step in this process is mechanical processing.

It is important to grind large particles into small pieces by chewing in order to increase their surface areas and allow for more efficient chemical reactions. It is also important to add water to the food, which further disperses the particles and provides the watery environment needed for these chemical reactions. Materials must also be mixed so that all the molecules that need to interact with one another have a good chance of doing so. The oral cavity and the stomach are the major body regions involved in cutting and grinding food to increase its surface area. This is another example of the surface area-to-volume concept presented at the beginning of this chapter. The watery mixture that is added to the food in the oral cavity is known as saliva, and the threee pairs of glands that produce saliva are known as salivary glands, saliva contains the enzyme salivary amylase, which initiates the chemical breakdown of starch. Saliva also lubricates the oral cavity and helps bind food before swallowing.

In addition to having taste buds that help identify foods, the tongue performs the important service of helping position the food between the teeth and pushing it to the back of the throat for swallowing. The oral cavity is very much like a food processor in which mixing and grinding take place. Figure 18.12 describes and summarizes the s of these structures.

Once the food has been chewed, it is swallowed and passs down the esophagus to the stomach. The process of swallowing ivolves a complex series of events. First, a ball of food, known as a bolus, is formed by the gongue and moved to the back of the mouth cavity. Here it stimulates the walls of the throat, also known as the pharyx. Nerve endings in the lining of the phyrynx are stimulated, causing a reflex contraction of walls of the esophagus, which transports the bolus to the stomach. Because both food and air  pass through the pharynx, it is important to prevent food from getting into the lungs. During swallowing the larynx is pulled upward. This causes a flap of tissue called the epiglottis to cover the opening to the trachea and prevent food from entering the trachea. In the stomach, additional liquid, called gastric juice, is added to the food. Gastric juice contains enzyems and hydrochloric acid. The major enzyme of the stomach is pepsin, which initiates the chemical breakdown of protein. The pH of gastric juice is very low, generally around pH 2. consequently, very few kinds of bacteria or protozoa emerge from the stomach alive. Those that do have special protective features that allow them to survive as they pass through the stomach. The entire mixture is churned by the contractions of the three layers of muscle in the stomach wall. The combined activities of enzymatic breakdown, chemical breakdown by hydrochloric acid, and mechanical processing by muscular movement result in a throughly mixed liquid called chyme. Chyme eventually leaves the stomach through a valve known as the pyloric sphincter and enters the small intestine.

The first part of the small intestine is known as the duodenum. In addition to producing enzymes, the duodenum secretes several kinds of hormones that regulate the release of food from the stomach and the release of secretions from the pancreas and liver. The pancreas produces a number of different digestive enzymes and also secretes large amounts of bicarbonate ions, which neutralize stomach acid so that the pH of the duodenum is about pH 8. the liver is a large organ in the upper abdomen that performs several s. One of its s is the secretion of bile. When bile leaves the liver, it is stored in the gallbladder prior to being released into the duodenum. When bile is released from the gallbladder, it assists mechanical mixing by breaking large fat globules into smaller particles. This process is called emulsification.

Emulsification is important because fats are not soluble in water, yet the reactions of digestion must take place in a water solution. Bile causes large globules of fat to be broken into much smaller units (increasing the surface area-to-volume ratio) much as soap breaks up fat particles into smaller globules that are suspended in water and washed away. The activity of bile is important for the further digestion of fats in the intestine.

Along the length of the intestine, additiona watery juices are added until the mixture reaches the large intestine. The large intestine is primarily involved in reabsorbing the water that has been added to the food tube along with saliva, gastric juice, bile, pancreatic secretions and intestinal juices. The large intestine is also home to variety of different kinds of bacteria. Most live on the undigested food that makes it through the small intestine. Some provide additional benefit by producing vitamins that can be absorbed from the large intestine. A few kinds may cause disease.

Several different kinds of enzymes have been mentioned in this discussion. Each is produced by a specific organ and has a specific . Chapter 5 introduced the topic of enzymes and how they work. Some enzymes, such as those involved in glycolysis, the krebs cycle, and protein synthesis are produced and used inside cells; others, such as the digestive enzymes, are produced by cells and secreted into the digestive tract. Digestive enzymes are simply a special class of enzymes and have the same characteristics as the enzymes you studied previously. They are protein molecules that speed up specific chemical reactions and are sensitive to changes in temperature or pH. The various diegestive enzymes, the sites of their production, and their s are listed in table 18.2.

Nutrients uptake

The process of digestion results in a variety of simple organic molecules that are available for absorption from the tube of the gut into the circulatory system. As we move simple sugars, amino acids, glycerol, and fatty acids into the circulatory system, we encounter another situation where surface area is important. The amount of material that can be taken up is limited by the surface area available. This problem is solved by increasing the surface area of the intestinal tract in several ways. First, the small intestine is a very long tube; the longer the bube, the greater the intestinal surface area. In a typical adult human it is about 3 meters long. In addition to length, the lining of the intestine consists of millions of fingerlike projections called villi, which increase the surface area. When we examine the cells that make up the villi, we find that they also have folds in their surface membranes. All of these characteristics increase the surface area available for the transport of materials from the gut into the circulatory system. Scientists estimate that the cumulative effect of all of these features produces a total intestinal surface area of about 250 square meters. That is equivalent to about half the area of a football field.

The surface area by itself would be of little if it were not for the intimate contact of the circulatory system with this lining. Each villus contains several capillaries and a branch of the lymphatic system called a lacteal. The close association between the intestinal surface and the circulatory system and the lymphatic systems allows for the efficient uptake of nutrients from the cavity of the gut into the circulatory system.

Several different kinds of processes are involved in the transport of materials from the intestine to the circulatory system. Some molecules such as water and many icons, simply diffuse through the wall of the intestine into the circulatory system. Other materials, such as amino acids and simple sugars, are assisted across the membrane by carrier molecules. Fatty acids and glycerol are absorbed into the intestinal lining cells where they are resynthesized into fats and enters lacteals in the villi. Because the lacteals are part of the lymphatic system, which eventually empties its contents into the circulatory system, fats are also transported by the blood. They just reach the blood by a different route.

Chemical alteration: the role of liver

When the blood leaves the intestine, it flows directly to the liver through the hepatic portal vein. Portal veins are blood vessels that collect blood from capillaries in one part of the body and deliver it to a second set of capillaries in another part of the body without passing through the heart. Thus the hepatic portal vein collects nutrient-rich blood from the intestine and delivers it directly to the liver. As the blood flows through the liver, enzymes in the liver cells modify many of the molecules and particles that enter them. One of the s of the liver is to filter any foreign organisms from the blood that might have entered through the intestinal cells. It also detoxifies many dangerous molecules that might have entered with the food.

Many foods contain toxic substances that could be harmful if not destroyed by the liver. Ethy alcohol is one obvious example. Many plants contain various kinds of toxic molecules that are present in small quantities and could accumulated to dangerous levels if the liver did not perform its role of detoxification.

In addition, the liver is responsible for modifying nutrient molecules. The liver collects glucose molecules and synthesizes glycogen, which can be stored in  the liver for later use. When glucose is in short supply, the liver can convert some of its stored glucogen back into glucose. Although amino acids are not stored, the liver can change the relative numbers of different amino acids circulating in the blood. It can remove the amino group from one kind of amino acid and attach it to a different carbon skeleton, generating a different amino acid. The liver is also able to take the amino acid group off amino acids so that what remains of the amino acid can be used in aerobic respiration. The toxic amino groups are then converted to urea by the liver. Urea is secreted back into the bloodstream and is carried to the kidneys for disposal in the urine.

18.5 Waste disposal

Because cells are modifying molecules during blic processes, harmful waste products are constantly being formed. Urea is a common waste; many other toxic materials must be eliminated as well. Among these are large numbers of hydrogen ions produced by bolism. This excess of hydrogen ions must be removed from the blood stream. Other molecules, such as water and salts, may be consumed in excessive amounts and must be removed. The primary organs involved in regulating the level of toxic or unnecessary molecules are the kidneys.

Kidney structure

The kidneys consist of about 2.4 million tiny units called nephrons. At one end of a nephron is cup-shaped structure called Bowman’s capsule, which surrounds a knot of capillaries known as glomerulus. In addition to Bowman’s capsule, a nephron consists of three distinctly different regions: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. The distal convoluted tubule of a nephron is connected to a collecting duct that transports fluid to the  ureters, and ultimately to the urinary bladder, where it is stored until it can be eliminated.

Kidney

As in the other systems discussed in this chapter,the excretory system involves a close connection between the circulatory system and a surface. in this case the large surface is provided by the walls of the millions of nephrons, which are surrounded by capillaries. Three major activities occur at these surfaces: filtration, reabsorption, and secretion. The glomerulus presents a large surface for the filtering of material from the blood to Bowman’s capsule. Blood that enters the glomerulus is under pressure from the muscular contraction of heart. The capillaries of the glomerulus are quite porous and provide a large surface area for the movement of water and small dissolved molecules from the blood into Bowman’s capsule. Normally, only the smaller molecules, such as glucose, amino acids, and ions, are able to pass through the glomerulus into the bowman’s capsule at the end of the nephron. The various kinds of blood cells and larger molecules like proteins do not pass out the blood into the nephron. This physical filtration process allows many kinds of molecules to leave the blood and enter the nephron. The volume of material filtered in this way through the approximately 2.4 million nephrons of our kidneys is about 7.5 liters per hour. Because your entire blood supply is about 5 to 6 liters, there must be some method of recovering much of this fluid.

     Surrounding the various portions of the nephron capillaries that passively accept or release molecules on the basis of diffusion gradients. The wall of the nephron are made of cells that actively assist in the transport of materials. Some molecules are reabsorbed from the nephron and picked up by the capillaries that surround them, whereas other molecules are actively secreted into the nephron from the capillaries. Each portion of the nephron has cells with specific secretory abilities.

     The proximal convoluted tubule is primarily responsible for reabsorbing valuable materials from the fluid moving through it. Molecules like glucose, amino acids, and sodium ions are actively transported across the membrane of the proximal convoluted tubule and returned to the blood. In addition, water moves across the membrane because it follows the absorbed molecules and diffuses to area where water molecules are less common. By the time the fluid has reached the end of the proximal convoluted tubule, about 65% of the fluid has been reabsorbed into the capillaries surrounding this region.

   The next portion of the bubule, the loop of Henle, is primarily involved in removing additional water from the nephron. Although the details of the mechanism are complicated, the principles are rather simple. The cells of the ascending loop of Henle actively transport sodium ions from the nephron into the space between nephrons where sodium ions accumulate in the fluid that surrounds the loop of Henle. The collecting ducts pass through this region as they carry urine to the ureters. Because the area these collecting ducts pass through is high in sodium ions, water within the collecting ducts diffuses from the ducts and is picked up by surrounding capillaries. However, the ability of water to pass through the wall of collecting duct is regulated by hormones. Thus it is possible to control water loss from the body by regulating the amount of water lost from the collecting ducts. For example. If you drank a liter of water or some other liquid, the excell water would not be allowed to leave the collecting duct and would exit the body as part of the urine. However, if you were dehydrated, most of the water passing through the collecting ducts would be reabsorbed, and very little urine would be produced. The primary hormone involved in regulating water loss is antidiuretic hormone. When the body has excess water, cells in the hypothalamaus of the brain respond and send a signal to the pipuitary and only a small amount of ADH is released and water is lost. When you are dehydrated these same brain cells cause more ADH to be released and water leaves the collecting duct and is returned to the blood.

The distal convoluted tubule is primarily involved in fine-tuning the amounts of various kinds of molecules that are lost in the urine. Hydrogen ions, sodium ions, chloride ions, potassium ions, and smmonium ions are regulated in this way.

    Some molecules that pass through the nephron are relatively unaffected by the various activities going on in the kidney. One of these is urea, which is filtered through the glomerulus into Bowman’s capsule. As it passes through the nephron, much of it stays in the tubule and is eliminated in the urine. Many other kinds of molecules , such as minor bolic waste products and some drugs, are also treated in this manner. Figure 18.16 summarizes the major s of the various portions of the kidney tubule system.

Behavior is how an animal acts, what it does, and how it does it. When we observe behavior, we must keep in mind that behavior is adaptive just like any other characteristic displayed by an animal. Behaviors are important for survival and appropriate for the environment in which an animal lives. As with the highly adaptive structural characteristics we see in animals, behaviors are the result of a long evolutionary process.

Both plants and animals must solve the same kinds of problems. For example, they must obtain nutrients, avoid being eaten, and reproduce. While behaviors are important for animals, plants, for the most part must rely on structures physiological changes, or chance to accomplish the same ends. Animals use many kinds of behaviors (searching, flying, walking, and chewing) to find and process the food they need, while plants can only obtain nutrients that happen to be located near their roots. A rabbit can run away from a predator, a plant cannot. But plants may have thorns or toxic compounds in their leaves that discourage animals from eating them. Animals engage in elaborate courtship behaviors to identify the species and sex of a potential mate. Most plants rely on a much more random method for transferring male gametes to the female plant.

Before we go much further, we need to discuss how animals generate specific behaviors and the two major kinds of behaviors: instinctive and learned. Both instinct and learning are involved in the behavior patterns of most organisms. We recognize instinctive behavior as behavior that is inborn, automatic, and inflexible; whereas learned behavior requires experience, produces new behaviors and can be changed. Most animals have a high proportion of instinctive behavior and very little learning; some, like many birds and mammals, are able to demonstrate a great deal of learned behavior in addition to many instinctive behaviors. Since majority of the behaviors of most animals are instinctive, inherited behaviors, they evolved just like the structures of animals. In this respect, behavioral characteristics are no different from structural characteristics. However, the evolution of behavior is much more difficult to find fossils that allow us to follow the development of behavior the way allow us to follow changes in structures. Fossils of footprints, and nests, and the presence of specific structures like grinding teeth, do tell us something about the behavior of extinct animals, but these represent only a few fragments of the total behavior that must have been part of the life of extinct animals. When we compare the behavior of closely related animals living today, it is often possible to see inherited behaviors that are slightly different from one another, just as the wings of different species of birds are slightly different but based on the same pattern.

17.2 interpreting behavior

When you watch a bird or squirrel or any other animal, its activities appear to have a purpose. Birds search for food, take flight as you approach, and build nests in which to raise young. Usually the nests are inconspicuous or placed in difficult-to-reach spots. Likewise, squirrels collect and store nuts and acorns, “scold” you when you get too close, and learn to visit sites where food is available. All of these activities are adaptive and contribute to the success of the species. Birds that do not take flight at the approach of another animal will be eaten by predators. Squirrels that do not find deposits of food will be less likely to survive, and birds that build obvious nests on the ground will be more likely to lose their young to predators. However, we need to take care not to attach too much meaning to what animals do. They may not have the same “thoughts” and “motivations” we do.

Why are most people afraid of snakes? Is this a behavior we are born with or do we learn it? It appears that fear of snakes is a learned behavior in humans because little children do not react to snake any differently than to other small moving objects. Fear of snakes can certainly be a valuable behavior since many common kinds of snakes are poisonous. Why do you blink when an object rapidly approaches your face? The automatic blinking of eyes is a reflexive behavior that is programmed into your nervous system. The behavior serves to protect the eye from injury. Why do you find it difficult to communicate with someone on the phone or by computer than face-to-face? You probably find it more difficult to communicate when you cannot see the person because you rely on facial expression and gestures to communicate part of the message.

It is not always easy to identify the significance of a behavior without careful study of the behavior pattern and the impact it has on other organisms. For example, a hungry baby herring gull pecks at a red spot on its parent’s bill. What possible can this behavior have for either the chick or the parent? If we watch, we see that when the chick pecks at the spot, the parent regurgitates food onto the ground, and the chick feeds. This looks like a simple behavior, but there is more to it than meets the eye. Why did the chick peck to begin with? How did it know to peck at that particular spot? Why did the pecking cause the parent to regurgitate food? These questions are not easy to answer. Many people assume that the actions have the same motivation and direction as similar human behaviors, but this is not necessarily a correct assumption. For example, when a human child points to a piece of candy and makes appropriate noises, it is indicating to its parent that it wants some candy. Is that what the herring gull chick is doing? We don’t know. Although both kinds of young may get food, we don’t know what the baby gull is thinking because we can’t ask it.

17.3 The problem of anthropomorphism

Poets, composers, and writers have often described birdsong as the act of a joyful songster. But is that bird singing on a warm, sunny spring day making that beautiful sound because it is so happy? Students of animal behavior do not accept this idea and have demonstrated that a bird sings to tell other birds to keep out of its territory. The barbed stinger of a honeybee remains in your skin after you are stung, and the bee tears the stinger out of its body when it flies away. The damage to its body is so great that it dies. Has the bee performed a noble deed of heroism and self-sacrifice? Was it defending its hive from you? We need to know a great deal more about the behavior of bees to understand the of such behavior to the success of the bee species. The fact that bee are social animals like us makes it particularly tempting to think that they are doing the same things for the same reasons we are.

The idea that we can ascribe human feelings, meanings, and emotions to the behavior of animals is called anthropomorphism. But we cannot crawl inside the brain of another animal and see what it is thinking so we need to avoid these anthropomorphic assumptions when attempting to understand the behavior of nonhuman animals. The fable of the grasshopper and ant is another example of inappropriately crediting animals with human qualities. The ant is pictured as an animal that, despite temptations, works hard from morning until night, storing away food for the winter. The grasshopper, on the other hand, is represented as a lazy good-for-nothing that fools away the summer singing, when it really ought to be saving up for the tough times ahead. If one is looking for analogies to human behavior, these are pretty good illustrations. But they really are not accurate statements about the lives of the animals from the biological point of view. Ant colonies may live through the winter on stored food and all the grasshoppers may die. However, the grasshopper has left eggs in the sheltered spots and these will hatch the next spring. Both the ant and grasshopper are very successful organism, but each has a different way of satisfying its needs and ensuring that some of its offspring will be able to provide another generation of organisms. One method of survival is not necessarily better than another, as long as both animals are successful. This is what the study of behavior is all about—looking at the activities of an organism during its entire life and determining the of the behavior in the ecological niche of the organism. The scientific study of the nature of behavior and its ecological and evolutionary significance in its natural setting is known as ethology.

17.4 instinct and learning

Instinctive behaviors are automatic, preprogrammed, and genetically determined. Such behaviors are found in a wide range of organism from simple one-celled protozoans to complex vertebrates. These behaviors are preformed correctly the first time without previous experience when the proper stimulus is given. A stimulus is some change in the internal or external environment of the organism that causes it to react. The reaction of the organism to the stimulus is called a response.

An organism can respond only to stimuli it can recognize. For example, it is difficult for us as humans to appreciate what the world seems like to a bloodhound. The bloodhound is able to identify individuals by smell, whereas we have great difficulty detecting, let alone distinguishing, many odors. Some animals, like dogs, deer, and mice are color-blind and are able to see only shades of gray. Others, such as honeybees, see ultraviolet light, which is invisible to us. Some birds and other animals are able to detect the magnetic field of the Earth. And some, such as rattlesnakes, are able to detect infrared radiation (heat) from distant objects.

In our example of the herring gull chick, the red spot on the bill of the adult bird serves as a stimulus to the chick. The chick responds to this spot in a genetically programmed way. The behavior is innate—it is done correctly the first time without prior experience. The pecking behavior of the chick is in turn the stimulus for the adult bird to regurgitate food. It is obvious that these behaviors have adaptive for the gull species because the leave little to chance. The young will get food from the adult automatically. Instinctive behaviors have great to the species because it allows correct, appropriate, and necessary behavior to occur without prior experience.

The drawback of instinctive behavior is that it cannot be modified when a new situation presents itself, but it can be very effective for the survival of a species if it is involved in fundamental, essential activities that rarely require modification. Instinctive behavior is most common in animals that have short life cycles, simple nervous systems, and little contact with parents. Over long periods of evolutionary time, these genetically determined behaviors have been selected for and have been useful to most of the individuals of the species. However, some instances of inappropriate behavior may be generated by unusual stimuli or unusual circumstances in which the stimulus is given. For example, many insects fly toward a source of light. Over the million of years of insect evolution, this has been a valuable and useful behavior. It allows them to easily find their way to open space. However, the human species invented artificial lights and transparent windows that generate totally inappropriate behavior. You have seen insects drawn to lights at night or observed insects inside house constantly flying against window panes through which sunlight is entering. This mindless, mechanical behavior seems incredibly stupid to us but, although individual animal die, the behavior is still valuable for the species because the majority do not encounter artificial lights or get trapped inside houses, and complete their life cycles normally.

Geese and other bird sit on nests to keep the eggs warm. During this incubation period, eggs may roll out of the nest as the parents get on and off the nest. When this happens, certain species of geese display a behavior that involves rolling eggs back into the nest. If the developing young within the egg are exposed to extremes of heat or cold they may be killed. Thus the egg-rolling behavior has a significant adaptive . If, however, the egg is taken from the goose when it is in the middle of egg-rolling behavior, the goose will continue its egg rolling until it gets back to the nest, even though there is no egg to roll. This is typical of the inflexible nature of instinctive behaviors. It was also discovered that many other somewhat egg-shaped structures would generate the same behavior. For example, beer cans and baseballs were good triggers for egg-rolling behavior. So not only was the bird unable to stop the egg-rolling behavior in midstride, but several nonegg objects generated inappropriate behavior because they had approximately the correct shape.

Some activities are so complex that it seems impossible for an organism to be born with such abilities. For example, you have seen spiderwebs in field, parks, or vacant lots. You may have even watched a spider spin a web. This is not jus a careless jumble of silk threads. A web is so precisely made that you can recognize what species of spider made it. But web spinning is not a learned ability. A spider has no opportunity to learn how to spin a web because it never observes others doing it. Furthermore, spiders do not practice several times before they get a proper, workable web. It is as if a “program” for making a particular web is in the spider’s “computer”. Many species of spiders appear to be unable to repair defective webs. When a web is damaged they typically start from the beginning and build an entirely new web. This inability to adapt as circumstances change is a prominent characteristic of instinctive behavior.

Could these behavior patterns be the result of natural selection? It is well established that many of kinds of behaviors are controlled by genes. The “computer” in out example is really the dna of the organism, and the “program” consists of a specific package of genes. Through the millions of years that spiders have been existence, natural selection has modified the web-making program to refine the process. Certain genes of the program have undergone mutation, resulting in changes in behavior. Imagine various ancestral spiders, each with a slightly different program. The inherited program that give the best chance of living long enough to produce a new generating is the program selected for and most likely to be passed on to the next generation.

Learned behavior

The alternative to programmed, instinctive behavior is learned behavior. Learning is a change in behavior as a result of experience. (Your behavior will be different in some way as result of reading this chapter.)

Learning becomes more significant in long-lived animals that care for their young. Animals that live many years are more likely to benefit from an ability to recognize previously encountered situations and modify their behavior accordingly. Furthermore, because the young spend time with their parents they can imitate their parents and develop behaviors that are appropriate to local conditions. These behaviors take time to develop but have the advantage of adaptability. In order for learning to become a dominant feature of an animal’s life, the animal must also have a memory which requires a relatively large brain in which to store the new information it is learning. This is probably why learning is a major part of life for only a few kinds of animals like the vertebrates. In humans, it is clear that nearly all behavior is learned. Even such important behaviors as walking, communicating, feeding oneself, and sexual intercourse must be learned.

17.5 kinds of learning

Scientists who study learning recognize that there are different kinds that can be subdivided into several categories: habituation, association, imprinting, and insight.

Habituation

Habituation is a change in behavior in which an animal ignores an insignificant stimulus after repeated exposure to it.

There are many examples of this kind of learning. Typically, the animals flee from humans. Under many conditions this is a valuable behavior. However, in situations where wild animals frequently encounter humans and never experience negative outcomes, they may “learn” to ignore humans. Many wild animals such as the deer, elk, and bears in parks have been habituated to the presence of humans and behave in a way that would be totally inappropriate in areas near the park where hunting is allowed. Similarly, loud noises will startle humans and other animals. However constant exposure to such sounds results in the individuals ignoring the sound. As a matter of fact, the sound may become so much a part of the environment that the cessation of the sound will evoke a response. This kind of learning is valuable because the animal does not waste time and energy responding to a stimulus that will not have a beneficial or negative impact on the animal. Animals that are continually responding to inconsequential stimuli have less time to feed and may miss other more important stimuli.

Association

Association occurs when an animal makes a connection between a stimulus and an outcome. Associating a particular outcome with a particular stimulus is important to survival because it allows an animal to avoid danger or take advantage of a beneficial event. If this kind of learning allows the animal to get more food, avoid predators, or protect its young more effectively, it will be advantageous to the species. The association of certain shapes, colors, odors, or sounds with danger is especially valuable. There are three common kinds of association learning; classical conditioning, operant (instrumental) conditioning, and observational learning (imitation).

Classical conditioning

Classical conditioning occurs when an involuntary, natural reflexive response to a natural stimulus is transferred from the natural stimulus to a new stimulus. The response produced by the new stimulus is called a conditioned response. During the period when learning is taking place, the new stimulus is given before or at the same time as the normal stimulus.

A Russian physiologist, Ivan Pavlov (1849-1936), was investigating the physiology of digestion when he discovered that dogs can transfer a normal response to a new stimulus. He was studying the production of saliva by dogs and he knew that a natural stimulus, such as the presence or smell of food, would cause the dogs to start salivating. Then he rang a bell just prior to the presentation of the food. After a training period, the dogs would begin to salivate when the bell was rung even though no food was presented. The natural response (salivating) was transferred from the natural stimulus (smell or taste of food) to the new stimulus ( the sound of a bell). Animals can also be conditioned unintentionally. Many pets anticipate their mealtimes because their owners go through a certain set of behaviors, such as going to a cupboard or opening a can of pet food prior to putting food in the dish. It is doubtful that this kind of learning is a common occurrence in wild animals, because it is hard to imagine such tightly controlled sets of stimuli in nature.

Operant conditioning

Operant (instrumental) conditioning also involves the association of a particular outcome with a specific stimulus, but differs from classical conditioning in several ways. First, during operant conditioning the animal learns to repeat acts that bring good results and avoid those that bring bad results. Second, a reward or punishment is received after the animal has engaged in a particular behavior. Third, the response is typically more complicated behavior than a simple reflex. A reward that encourages a behavior is known as positive reinforcement and a punishment that discourages a behavior is known as negative reinforcement. The training of many kinds of animas involves this kind of conditioning. If a dog being led on a chain is given the command “heel” and is then vigorously jerked into the correct position by its master it eventually associates the word “heel” with assuming the correct position at the knee. This is negative reinforcement because the animal avoids the unpleasantness of being jerked about if it assumes the correct position. Similarly, petting or giving food to a dog when it has done something correctly will positively reinforce the desired behavior. For example, pushing the dog into the sitting position on the command “sit” and rewarding the dog when it performs the behavior on command is positive reinforcement.

Wild animals have many opportunities to learn through positive or negative reinforcement. As animals encounter the same stimulus repeatedly there is an opportunity to associate the stimulus with a particular outcome. For example, many kinds of birds eat barriers and other small fruits. If a distinctly colored berry has a good flavor, birds will return repeatedly and feed from that source. Pigeons in cities have learned to associate food with people in parks. They can even identify specific individuals who regularly feed them. Their behavior is reinforced by being fed. Many birds in urban areas have associated automobiles with food, and are seen picking smashed insects from the grills and bumpers of cars. When a car drives into the area it is immediately examine for food.

In some of our national parks, bears have associated backpacks with food. In some case, attempts have been made to use negative reinforcement to condition these bears to avoid humans. Bears that are repeat offenders are often killed. Conversely, in areas where bears are hunted they have generally been conditioned to avoid contact with humans. If certain kinds of fruits or insects have unpleasant tastes, animals will learn to associate the bad tastes with the colors and shapes of the offending objects and avoid them in the future. Each species of animal has a distinctive smell. If a deer or rabbit has several bad experiences with a predator that has s particular smell, it can avoid places where the smell of the predator is present.

Animals also engage in trial-and-error learning which involves elements of conditioning. When confronted with a particular problem, they will try one option after another until they achieve a positive result. Once they have solved the problem, they can use the same solution repeatedly. For example, if a squirrel has a den in a hollow tree on one side of a stream and is attracted to a source of food on the other side, it may explore several routes to get across the stream. It may jump from a tree on one side of the stream to another on the opposite side. It may run across a log that spans the stream. It may wade a shallow portion of the stream. Once it has found a good pathway, it is likely to use the same pathway repeatedly. Many hummingbirds visit many different flowers during the course of a day. When they have found a series of nectar-rich flowers, they will follow a particular route and visit the same flowers several times a day.

Observational learning

In animals that participate in social groups, imitation is possible. Observational learning (imitation) is a form of associative learning that consists of a complex set of associations formed while watching another animal being rewarded or punished after performing a particular behavior. In this case, the animal is not receiving the reward or punishment itself but is observing the “fruits” of the behavior of other animals. Subsequently, the observer may show the same behavior. It is likely that conditioning is involved in imitation, since when an animal imitates a beneficial behavior it is rewarded. Observing a negative outcome to another animal is also beneficial because it allows the observer to avoid negative consequences. Many kinds of young birds and mammals follow their parents and sample the same kind of foods their parents eat. If the food tastes good, they are positively reinforced. They may also observe warning and avoidance behaviors associated with particular predators and mimic these behaviors when the predator is present. For example, crows will “mob” predators such as hawks and owls. As young birds observe older crows cawing loudly and chasing an owl, the young crows learn to perform the same behavior. They associate a certain kind of behavior with a certain kind of stimulus.

Exploratory learning

Animals are constantly moving about and sampling their environment. Since they have a memory, it is possible for animals to store information about their surroundings as they wander about. In some cases, the new information may have immediate . For example, in the spring of the year a queen bumblebee will fly about examining holes in the ground. Eventually she will find a hole in which she will lay eggs and begin to raise her first brood of young. Once she has selected a site she must learn to recognize that particular spot so she can return to it each time she leaves to find food, or her young will die. In similar fashion, the exploratory behavior of birds and mammals allows them to find sources of food which they can return repeatedly. When you put up a bird feeder, it does not take very long before many birds are visiting the feeder on a regular basis.

     In other cases, the information learned may not be used immediately but could be of use in the future. If an animal has an inventory of its environment, it can call on the inventory to solve problems later in life. Many kinds of animals hide food items when food is plentiful and are able to find these hidden sources later when the food is needed. Even if they don’t remember exactly where the food is hidden, if they always hide food in a particular kind of place they are likely to be able to find it at a later date. (for example, if you need to drive a car that you have never seen before, you would know that you need to use the key and you would search in a particular place in the car for the place to insert the key.) having a general knowledge of its environment is very useful to an animal.

Many kinds of small mammals such as mice and ground squirrels avoid predators by scurrying under logs or other objects or into holes in the ground. Experiments with mice and owl predators show that mice that have developed a familiarity with their surrounding are more likely to escape predators than are those that are unfamiliar with their surroundings.

Imprinting

Imprinting is a special kind of irreversible learning in which a very young animal is genetically primed to learn a specific behavior in a very short period during a specific time in its life. The time during which the learning is possible is known as the critical period. This type of learning was originally recognized by konrad Lorenz in his experiments with geese and ducks. He determined that, shortly after hatching, a duckling would follow an object if the object was fairly large, moved, and made noise. In one of his books, Lorenz described himself as squatting on the lawn one day, waddling and quacking, followed by newly hatched ducklings. He was being a “mother duck”. He was surprised to see a group of tourists on the other side of the fence watching his in amazement. They couldn’t see the ducklings hidden by the tall grass. All they could see was this strange performance by a big man with a beard!

Ducklings will follow only the object on which they were originally imprinted. Under normal conditions, the first large, noisy, moving object newly hatched ducklings see is their mother. Imprinting ensures that the immature birds will follow her and learn appropriate feeding, defensive tactics, and other behaviors by example. Because they are always near their mother, she can also protect them from enemies or bad weather. If animals imprint on the wrong objects, they are not likely to survive. Since these experiments by Lorenz in the early 1930s, we have discovered that many young animals can be imprinted on several types of stimuli and that there are responses other than following.

The way song sparrows learn their song appears to be a kind of imprinting. It has been discovered that the young birds must hear the correct song during a specific part of their youth or they will never be able to perform the song correctly as adults. This is true even if later in life they are surrounded by other adult song sparrows that are singing the correct song. Furthermore, the period of time when they learn the song is prior to the time they begin singing. Recognizing and performing the correct song is important because it has particular meaning to other song sparrows. For males, it conveys the information that a male song sparrow has a space reserved for himself. For females, the male’s song is an announcement of the location of a male of the correct species that could be a possible mate.

Mother sheep and many other kinds of mammals imprint on the odor of their offspring. They are able to identify their offspring among a group of lambs and will allow only their own lambs to suck milk. Shepherds have known for centuries that they can sometimes get a mother that has lost her lambs to accept an orphan lamb if they place the skin of the mother’s dead lamb over the orphan.

Many fish appear to imprint on odors in the water. Salmon are famous for their ability to return to the freshwater streams where they were hatched. They will jump waterfalls and use specially constructed fish ladders to get around dams. Fish that are raised in artificial hatcheries can be imprinted on minute amounts of special chemicals and be induced to return to any stream that contains the chemical.

Insight.

Insight is a special kind of learning in which past experiences are reorganized to solve new problems. When you are faced with a new problem, whether it is a crossword puzzle, a math problem, or any one of a hundred other everyday problems, you sort through your past experiences and locate those that apply. You may not even realize that you are doing it, but you put these past experiences together in a new way that may give the solution to your problem. Because this process is internal and can be demonstrated only through some response, it is very difficult to understand exactly what goes on during insight learning. Behavioral scientists have explored this area for many years, but the study of insight learning is still in its infancy.

Insight learning is particularly difficult to study because it is impossible to know for sure whether a novel solution to a problem is the result of “thinking it through” or an accidental occurrence. For example, a small group of Japanese macaques (ms) was studied on an island. They were fed by simply dumping food, such as sweet potatoes or wheat, onto the beach. Eventually, one of the macaques discovered that she could get the sand off the sweet potato by washing it in a nearby stream. She also discovered that she could sort the wheat from the sand by putting the mixture into water because the wheat would float. Are these examples of insight learning? We will probably never know, but it is tempting to think so. In addition, in the colony of macaques the other individuals soon began to display the same behavior, probably because they were imitating the female that first made the discovery.

Table 17.1 summarizes the significance of each of the kinds of learning.

17.6 instinct and learning in the same animal

It is important to recognize that all animals have both learned and instinctive behaviors and that one behavior may have elements that are both instinctive and learned. For example, biologists have raised young song sparrows in the absence of any adult birds so there was no song for the young birds to imitate. These isolated birds would sing a series of notes similar to the normal song of the species, but not exactly correct. Birds from the same nest that were raised with their parents developed a song nearly identical to that of their parents. If bird songs were totally instinctive, there would be no difference between these two groups. It appears that the basic melody of the song was inherited by the birds and that the refinements of the song were the result of experience. Therefore, the characteristic song of that species was partly learned behavior (a change in behavior as a result of experience) and partly unlearned (instinctive). This is probably true of the behavior of many organisms; they show complex behaviors that are a combination of instinct and learning. It is important to note that many kinds of birds learn most of their songs with very few innate components. Mockingbirds are very good at imitating the songs of a wide variety of bird species found in their local region.

This mixture of learned and instinctive behavior is not the same for all species. Many invertebrate animals rely on instinct for the majority of their behavior patterns, whereas many of the vertebrates (particularly birds and mammals) make use of a great deal of learning.

Typically the learned components of an animal’s behavior have particular for the animal’s survival. Most of the behavior of honeybee is instinctive, but it is able to learn new routes to food sources. The style of nest built by a bird is instinctive, but the skill with which it builds may improve with experience. The food-searching behavior of birds is probably instinctive, but the ability to modify the behavior to exploit unusual food sources such as bird feeders is learned. On the other hand, honeybees cannot be taught to make products other than honey and beeswax, a robin will not build a nest in a birdhouse, and most insect-eating birds will not learn to visit bird feeders. Table 17.2 compares instinctive behaviors and learned behaviors.

17.7 What about human behavior?

We tend to think of ourselves as being different from other animals, and we are. However, it is important to recognize that we are different only in the degree to which we employ these different kinds of behavior. Humans have few behaviors that can be considered instincts. We certainly have reflexes that cause us to respond appropriately without thinking. Touching a hot object and rapidly pulling your hand away is a good example. Newborns grasp objects and hang on tightly with both their hands and feet. This kind of grasping behavior in our primitive ancestors would have allowed the child to hang onto its mother’s hair as the mother and child traveled from place to place. But do we have more complicated instinctive behaviors? Although nearly all behavior other than reflexes is learned, newborn infants display several behaviors that could be considered instinctive. If you stroke the side of an infant’s face, the child will turn it head toward the side touched and begin sucking movements. This is not a simple reflex behavior but rather requires the coordination of several sets of muscles and certainly involves the brain. It is hard to see how this could be a learned behavior because the child does the behavior without prior experience. Therefore it is probably instinctive. This behavior may be associated with nursing, because carrying the baby on its back would place the check of the child against the breast of the mother. Other mammals, even those whose eyes do not open for several days following birth, are able to find nipples and begin nursing shortly after birth.

Habituation is a common experience. We readily ignore sounds that are continuous such as the sound of air conditioning equipment or the background music typical of shopping malls. Teachers recognize that it is important to change activities regularly if they are going to keep the attention of their students.

Associative learning is extremely common in humans. We associate smells with certain kinds of food, sirens with emergency vehicles, and words with their meanings. Much of the learning that we do is by association. We also often use positive and negative reinforcement as ways to change behavior. We seek to reward appropriate behavior and punish inappropriate behavior. Much of the positive and negative reinforcement can be accomplished without having the actual experience because we can visualize possible consequences of our behavior. Adults routinely describe consequences for children so that children will not experience particularly harmful effects. “if you don’t study for your biology exam you will probably fail it”.

Imprinting in human is more difficult to demonstrate, but there are some instances in which imprinting may be taking place. Bonding between mothers and infants is though to be an important step in the development of the mother-child relationship. Most mothers form very strong emotional attachments to their children and, likewise, the children are attached to their mothers, sometimes literally, as they seek to maintain physical contact with their mothers. However, it is very difficult to show what is actually happening at this early time in the life of a child.

Another interesting possibility is the language development of children. All children learn whatever languages are spoken where they grow up. If multiple languages are spoken they will learn them all and they learn them easily. However, adults have more difficulty learning new languages, and often they find it impossible to “unlearn” previous languages, so they speak new languages with an accent. This appears to meet the definition of imprinting. Learning takes place at a specific time in life, the kind of learning is pregrogrammed, and what is learned cannot be unlearned. Recent research using tomographic images of the brain shows that those who learned a second language as adults use two different parts of the brain for language—one part for the native language or languages they learned as children and a different part for their second language.

Insight is what our species prides itself on. We are thinking animals. Thinking is a mental process that involves memory, a concept of self, and an ability to reorganize information. We come up with new solutions to problems. We invent new objects, new languages, new culture, and new challenges to solve. However, how much of what we think is really completely new, and how much is imitation? As mentioned earlier, association is major core of our behavior, but we also are able to use past experiences, stored in our large brains, to provide clues to solving new problems.

17.8 selected topics in behavioral ecology

Of the examples used so far in this chapter, some involved laboratory studies, some were field studies, and some include aspects of both. Often these studies overlap with the field of psychology. This is particularly true for many of the laboratory studies. You can see that the science of animal behavior is a broad one that draws on information from several fields of study and can be used to explore many kinds of questions. The topics that follow avoid the field of psychology and concentrate on the significance of behavior from and ecological and an evolutionary point of view.

    Now that we have some understanding of how organisms generate behavior, we can look at a variety of behaviors in several kinds of animals and see how they are useful to the animals in their ecological niches.

Reproductive behavior

Reproductive behavior of many kinds of animals has been studied a great deal. Although each species of animal has its own specific behaviors, there are certain components of reproductive behavior that are common to nearly all kinds of animals. In order for an animal to reproduce, several events must occur—a suitable mate must be located, mating and fertilization must take place, and the young must be provided for.

Finding the other

In order to reproduce, an animal must find individuals of the same species that are of the opposite sex. Several techniques are used for this purpose. Different species of animals employ different methods, but most involve the production of signals that can be interpreted by others of the same species. Depending on the species, any of the senses (sound, sight, touch, smell, or taste) may be used to identify the species, sex, and sexual receptiveness of another animal. For instance, different species of frogs produce distinct calls. The call is a code system that delivers a very private message because it is meant for only one species. It is, however, meant for any member of that species near enough to hear. The call produced by male frogs, which both male and female frogs can receive by hearing, results in frogs of both sexes congregating in a limited area. Once they gather in a small pond. It is much easier to have the further communication necessary for mating to take place.

Many other animals including most birds, insects like crickets, reptiles like alligators, and some mammals produce sounds that are important for bringing individuals together for mating.

Chemicals can also serve to attract animals. Pheromones are chemicals produced by animals and released into the environment that trigger behavioral or developmental changes in other animals of the same species. They have the same effect as sound made by frogs or birds; they are just using a different code system. The classic example of a pheromone is the chemical that female moths release into the air. The large, fuzzy antenae of the male moths can receive the chemical in unbelievably tiny amounts. The male then changes his direction of flight and flies upwind to the source of the phromone, which is the female. Some of these sex-attractant pheomones have been synthesized in the laborary. One of these, called Disparlure, is widely used to attract and trap male gypsy moths. Because gypsy moths cause considerable damage to trees by feeding on the leaves, the sex attractant is used to estimate population size so that control measures can be taken to prevent large population outbreaks.

Most mammals rely on odors. Females typically produce distinct odors when they are in breeding condition. When males happen on such an odor trail, they follow it to the female. Many reptiles also produce distinctive odors.

Visual cues are also important for many species. Brightly colored birds, insects, fish, and many other animals often use specific patches of color for species identification. Conspicuous movements may also be used to attract the attention of a member of the opposite sex.

The firefly is probably the most familiar organism that uses light signals to bring males and females together. Several different species may live in the same area, but each species flashes its own code. The code is based on the length of the flashes, their frequency, and their overall pattern. There is also a difference between the signals given by males and those given by females. For the most part, males are attracted to and mate with females of their own species. Once male and female animals have attracted one another’s attention, the second stage in sucessful reproduction takes place. However, in one species of firefly, the female has the remarkable ability to signal the correct answering code to species other than her own. After she has mated with a male of her species, she will continue to signal to passing males of other species. She is not hungry for sex, she is just hungry. The luckless male who responds to her “come-on” is going to be her dinner.

Assuring fertilization

The second important activity in reproduction is fertilizing eggs. Many marine organism simply release their gametes into the sea simultaneously and allow fertilization and further development to take place without any input from the parents. Sponges, jellyfishes, and many other marine animals fit this category. Other aquatic animals congregate so that the chances of fertilization are enhanced by the male and female being near one another as the gametes are shed. This is typical of many fish and some amphibians, such as frogs. Internal fertilization, in which the sperm are introduced into the reproductive tract of the female, occurs in most terrestrial animals. Some spiders and other terrestrail animals produce package of sperm that the female picks up with her reproductive structures. Many of these mating behaviors require elaborate, species-specific communication prior to the mating act. Several examples were given in the previous paragraphs.

Raising the young

A third element in successful reproduction is providing the young wth the resources they need to live to adulthood. Many invertebrate animals spend little energy on the care of the young, leaving them to develop on their own. Usually the young become free-living larvae that eat and grow rapidly. In some species, females make preparations for the young by laying their eggs in suitable sites. Many insects lay their egges on particular species of plant that the larvae will use as food as it develops. Parasitic species seek out the required host in which to lay their eggs. The eggs of others may be place in spots that provide safety until the young hatch from the egg. Turtles, many fish, and some insects fit this category. In most of these cases, however, the female lays large numbers of eggs, and most of the young die before reaching adulthood. This is an enormously expensive process: the female invests considerable energy in the production of the eggs but has a low success rate.

     An alternative to this “wasteful” loss of potential young is to produce fewer young but invest large amounts of energy in their care. This is typical of birds and mammals. Parents build nests, share in the feeding and protection of the young, and often assist the young in learning appropriate hebaviors. Many insects, such as bees, ants, and termites, have elaborate social oragnizations in which one or a few females produce large numbers of young that are cared for by sterile offspring of the fertile females. Some of the female’s offspring wil be fertile, reproducing individuals.

The activity of caring for the young involves many complex behavior patterns. It appears that most animals that feed and raise young are able to recognize their own young from those of other nearby families and may even kill the young of another family unit. Elaborate greeting ceremonies are usually performe when animals return to the nest or the den. Perhaps this has something to do with being able to identify individual young. Often this behavior is shared among adults as well. This is true many colonial nesting birds such as gulls and penguins, and for many carnivorous mammals, such as wolves, dogs, and hyenas.

Territorial behavior

One kind of behavior pattern that is often tied to successful reproduction is territorial behavior. A territory is the space used for food, mating, or other purpose, that an animal defends against others of the same species. The behaviors involves in securing and defending the territory are called territorial behaviors. A territory has great importance because it reserves exclusive rights to the use of a certain space for an individual.

When territories are first being established, there is much conflict between individuals. This eventually gives way to the use of a series of signals that define the territory and communicate to others that the territory is occupied. The male redwing blackbird has red shoulder patches, but the female does not. The male will perch on a high spot, flash his red shoulder patches, and sing to other males that happen to venture into his territory. Most other males get the message and leave his territory; those that do not leave, he attacks. He will also attack a stuffed, dead male redwing blackbird in his territory, or even a small piece of red cloth. Clearly, the spot of red is the characteristic that stimulates the male to defend his territory. Once the initial period of conflict is over, the birds tend to respect one another’s boundaries. All that is required is to frequently announce that the territory is still occupied. This is accomplished by singing from sone conspicuous position in the territory. After the territorial boundaries are established, little time is required to prevent other males from venturing close. Thus the animal may spend a great deal of time and energy securing the territory initially, but doesn’t need to expend much to maintain it.

Not all male redwing blackbirds are successful to obtaining territories. During the initial period, when fighting is common, some birds regularly win and maintain their territories. Some lose and must choose a less favorable territory or go without. Therefore, territorial behavior is a way to distribute a resource that is in short supply. Because females choose which male’s territory they will build their nest in , males that do not have territories are much less likely to fertilize females.

With many kinds of animals the possession of a territory is often a requirement for reproductive success. In a way, then, terrtorial behavior has the effect of allocating breeding space and limiting population size to that which the ecosystem can support. This kind of behavior is widspread in the animal kindom and can be seen in such diverse groups as insects, spiders, fish, reptiles, birds, and mammals.

Many seabird colonies have extremely small nest territories. Each territory is just beyond the reach of the bills of the neighbors. Trespassers are severely punished. Within a gull colony, each nest is in a territory of about 1 square meter. When one gull walks or lands on the territory of another, the defender walks toward the other in the upright threat posture. The head is pointed down with the neck stretched outward and upward. The folded wings are raised slightly as if to be used as clubs. The upright threat posture is one of a number of movements that signal what an animal is likely to do in the near future. The bird is communicating an intention to do something, to fight in this case, but it may not follow through. If the invader shows no sign of retreating, then one or both gulls may start pulling up the grass very vigorously with their beaks. This seams to make no sense. The gulls were ready to fight one moment;  the next moment they apparently have forgotten about the conflict and are pulling grass. But the struggle has not been forgotten: pulling grass is an example of redirected aggression. In redirected aggression, the animal attacks something other than the natural opponent. If the intruding gull doesn’t leave at this point, there will be an actual battle. ( a person who pounds the desk during an argument is showing redirected aggression. Look for examples of this behavior in your neighborhood cats and dogs—maybe even in  yourself!)

Many carnivorous mammals like foxes, weasels, cougars, coyotes, and wolves use urine or other scents to mark the boundaries of their territories. One of the primary s of the territory for these animals is the food contained within the large space they defend. These territories may include several square kilometers of land. Many other kinds of animals are territorial but use other signaling methods to maintain ownership of their territories. For example, territorial fish use color patterns and threat postures to defend their territories. Crickets use sound and threat postures. Male bullfrogs engage in shoving matches to displace males who invade their small territories along the shoreline.

Dominance of hierarchy

Another way of allocating resources is by the establishment of a dominance hierarchy, in which a relatively stable, mutually understand order of priority within the group is maintained. A dominance hierarchy is often established in animals that form social groups. One individual in the group dominates all others. A second-ranking individual dominates all but the highest-ranking individual, and so forth, until the lowest-ranking individual must give way to all others within the group. This kind of behavior is seen in barnyard chickens, where it is known as a pecking order. Figure 17.11 shows a dominance hierarchy; the lead animal has the highest ranking and the last animal has the lowest ranking.

A dominance hierarchy allows certain individuals to get preferential treatment when resources  are scarce. The dominant individual will have first choice of food, mates, shelter, water, and other resources because of its position. Animals low in the hierarchy may be malnourished or fail to mate in times of scarcity. In many social animals, like wolves, usually only the dominant male and female reproduce. This ensures that the most favorable genes will be passed on to the next generation. Poorly adapted animals with low rank may never reproduce. Once a dominance hierarchy is established, it results in a more stable social unit with little conflict, except perhaps for an occasional atlercation that reinforces the knowledge of which position an animal occupies in the hierarchy. Such a hierarchy frequently results in low-ranking individuals emigrating from the area. Such migrating individuals are often subject to heavy predation. Thus the dominance hierarchy serves as a population-control mechanism and a way of allocating resources.

Avoiding periods of scarcity

Resource allocation becomes most critical during periods of scarcity. In some areas, the dry part of the year is the most stressful. In temperate areas, winter reduces many sources of food and forces organisms to adjust. Animals have several ways of coping with seasonal stress. Some animals simply avoid stress. In areas where drought occurs, many animals become inactive until water becomes available. Frogs, toads, and many insects remain inactive (estivate) underground during long periods and emerge to mate when it rains. Hibernation in warm-blooded animals is a response to cold, seasonal temperatures in which the body temperature drops and there is a physiological slowing of all body processes that allows an animal to survive on food it has stored within its body. Hibernation is typical of bats, marmots, and some squirrels. Similarly cold-blooded animals have their activities slowed because a drop in air temperature cause a corresponding drop in body temperature.

    Other animals have built-in behavior patterns that cause them to store food during seasons of plenty for periods of scarcity. These behaviors are instinctive and are seen in a variety of animals. Squirrels bury nuts, acorns, and other seeds. ( they also plant trees because they never find all the seeds they bury.) chickadees stash seeds in cracks and crevices when food is plentiful and spend many hours during winter exploring similar places for food. Some of the food they find is food they stored. Honeybees store honey, which allows them to live through the winter when nectar is not available. This requires a rather complicated set of behaviors that coordinates the acitivities of thousands of bees in the hive.

Navigation and migration

Because animals move from place to place to meet their needs it is useful to be able to return to a nest, water hole, den, or favorite feeding spot. This requires some sort of memory of their surroundings ( a mental map) and a way of determining direction. Often it is valuable to have information about distance as well. Direction can be determined by such things as magnetic fields, identifying landmarks, scent trails, or reference to the sun or stars. If the sun or stars are used for navigation, some sort of time sense is also needed because these bodies move in the sky.

In honeybees, navigation also involves communication among the various individuals that are foraging for nectar. The bees are able to communicate information about the direction and distance of the nectar source from the hive. If the source of nectar is some distance from the hive, the scout bee perform a “wagging dance” in the hive. The bee walks in a straight line for a short distance, wagging it rear end from side to side. It then circles around back to its starting position and walks the same path as before. This dance is repeated many times. The direction of the straight-path portion of the dance indicates the direction of the nectar relative to the position of the sun. for instance, if the bee walks straight upward on a vertical surface in the hive, that tells the other bees to fly directly toward the sun. if the path is 30 degrees to the right of vertical, the source of the nectar is 30 degrees to the right of the sun’s position.

The duration of the entire dance and the number of waggles in the straight-path portion of the dance are position correlated with the length of time the bee must fly to get to the nectar source. So the dance is able to communicate the duration of the flight as well as the direction. Because the recruited bees have picked up the scent of the nectar source from the dance, they also have information about the kind of flower to visit when they arrive at the correct spot. Because the sun is not stationary in the sky, the bee must constantly adjust its angle to the sun. it appears that they do this with some kind of internal clock. Bees that are prevented from going to the source of nectar or from seeing the sun will still fly in the proper direction sometime later, even though the position of the sun is different.

The ability to sense changes in time is often used by animals to prepare for seasonal changes. In areas away from the equator, the length of the day changes as the seasons change. The length of the day is called the photoperiod. Many birds prepare for migration and have their migration direction determined by the changing photoperiod. For example, in the fall of the year many birds instinctively change their behavior, store up fat, and begin to migrate from northern areas to area closer to the equator. This seasonal migration allows them avoid the harsh winter condition signaled by the shortening of days. The return migration in the spring is triggered by the lengthening photoperiod. This migration certainly requires a lot of energy, but t allows many birds to exploit temporary food resources in the north during the summer months.

Like honeybees, some daytime-migrating birds use the sun to guide them. We need two instruments to navigate by the sun—an accurate clock and a sextant for measuring the angle between the sun and the horizon. Can a bird perform such measurements without instruments when we, with our much bigger brains, need these instruments to help us? It is unquestionably true! For nighttime migration, some birds use the stars to help them find their way. In one interesting experiments, warbles, which migrate at night, were placed in a planetarium. The pattern of stars as they appear at any season could be projected onto a large domed ceiling. During autumn, when these birds would normally migrate south ward, the stars of the autumn sky were shown on the ceiling. The birds responded with much fluttering activity at the south side of the cage. As if they were trying to migrate southward. Then the experimenters tried porjecting the stars of the spring sky, even though it was autumn. Now the birds rended to try to fly northward, although there was less unity in their efforts to head north; the birds seemed somewhat confused. Nevertheless, the experiment showed that the birds recognized star patterns and were influenced by them.

There is evidence that some birds navigate by compass direction—that is, they fly as if they had a compass in their heads. They seem to be able to sense magnetic field. Their ability to sense magnetic fields was proven at the U.S. Navy’s test facility in Wisconsin. The weak magnetism radiated from this test site has changed the flight pattern of migrating birds. But it is yet to be proved that birds use the magnetism of the earth to guide their migration. Homing pigeons are famous for their ability to find their way home. They make use of a wide variety of clues, but it has been shown that one of the clues they use involves magnetism. Birds with tiny magnets glued to the sides of their heads were very poor navigators; others with nonmagnetic objects attached to the sides of their heads did not lose their ability to navigate.

Biological clocks

As mentioned earlier, bees, birds, and probably most other animals have internal clocks. In the case of bees, the clock allows them to predict the position of the sun. in the case of birds and mammals, the changing length of day allows them to time their migration, mating, food-storing behavior, or time for entering hibernation. So some clocks are annual clocks, whereas others are daily clocks. For instance, you have a daily clock. Travelers who fly partway around the world by nonstop jet plane need some time to recover from “jet lag”. Their digestion, sleep, or both, may be upset. Their discomfort is not caused by altitude, water, or food, but by having rapidly crossed several time zones. There is a great difference in the time as measured by the sun or local clocks and that measured by the body; the body’s clock adjusts more slowly.

    There are many examples of animals behaviors that are timed, some of which show a great deal of precision. In the animal world, mating is the most obviously timed event. In the pacific ocean, off some of the tropical islands, lives a marine worm known as the palolo worm. Its habit of making a well-timed brief appearance in enormous swarms is striking example of a biological clock phenomenon. At mating time, these worms swarm into the shallows of the islands and discharge sperm and eggs. There are so many worms that the sea looks like noodle soup. The people of the islands find this an excellent time to change their diets. They dip up the worms much as North Americans dip up smelt or other small fish that are making a spawning run. The worms appear around the third quarter of the moon in october or november, the time varying somewhat according to local environmental conditions. It appears that they have an annual cycle for mating but that a monthly, or lunar, cycle is superimposed on the annual cycle. Because these animals are marine worms it is unclear whether they are responding to the moon or the tidal effects of the moon or something else entirely.

Social behavior

Many species of animals are characterized by interacting groups called societies, in which there is division of labor. Societies differ from simple collections of organisms by the greater specialization and division of labor in the roles displayed by the individuals in the group. The individuals performing one cooperate with others having different special abilities. As a result of specialization and cooperation, the society has characteristics not found in any one member of the group: the whole is more than the sum of its parts. But if cooperation and division of labor are to occur, there must be communication among individuals and coordination of effect.

Honeybees, for example, have an elaborate communication system and are specialized for specific s. A few individuals known as queens and drones specialize in reproduction, where large numbers of worker honeybees are involved in collecting food, defending the hive, and caring for the larvae. These roles are rigidly determined by inherited behavior patterns. Each worker honeybee has a specific task, and all tasks must be fulfilled for the group to survive and prosper. As they age, the worker honeybees move through a series of tasks over a period of weeks. When they first emerge from their was cells, they clean the cells. Several days later, their job is to feed the larvae. Next they build cells. Later they become guards that challenge all insects that land near the entrance to the hive. Finally they become foragers who find and bring back nectar and pollen to feed the other bees in the hive. Foraging is usually the last job before the worker honeybee dies. Although this progression of tasks is the usual order, workers can shift from their main task to others if there is a need. Both the tasks performed and the progression of tasks are instinctively determined.

A hive of bees may contain thousands of individuals, but under normal conditions only the queen bee and the male drones are capable of reproduction. None of the thousands of workers who are also females will reproduce. This does not seem to make sense because they appear to be giving up their chance to reproduce and pass their genes onto the next generatioon. Is this some kind of self-sacrifice (altruistic behavior) on the part of the workers, or is there another explanations? In general, the workers in the hive are the darghters or sister of the queen and therefore share a large numbers of her genes. This means that they are really helping a portion of their genes get to the next generation by assisting in the raising of their own sisters, some of whom will become new queens. This argument has been used to partially explain behaviors in societies that might be bad for the individual but advantageous for the society as a whole.

Animal societies exhibit many levels of complexity and types of social organization differ from species to species. Some societis show little specialization of individualss other than that determined by sexual differences or differences in physical size and endurance. The African wild dog illustrates such a flexible social organization. These animals are nomadic and hunt in packs. Although an individual wild dog can kill prey about its own size, groups are able to kill fairly large animals if they cooperate in the chase and the kill, which often involves a chase of several kilometers. When the dogs are young, they do not follow the pack. When adults return from a successful hunt, they regurgitate food if the proper begging signal is presented to them. Therefore, the young and the adults that remained bebind to guard the young are fed by the hunters. The young are the responsibility of the entire pack, which cooperates in their feeding and protection. During the time that the young are at the den site, the pack must give up its nomadic way of life. Therefore, the young are born during the time of year when prey are most abundant. Only one or two of the females in the pack have young each year. If every female had young, the pack couldn’t feed them all. At about two months of age, the young begin to travel with the pack, and the pack can return to its nomadic way of life.

In many ways honeybee and African wild dog societies are similar. Not all females reproduce, the raising of young is a shared responsibility, and there is some specialization of roles. The analysis and comparison of animal societies has led to the thought that there may be fundamental process that shape all societies. The systematic study of all forms of social behavior, both human and nonhuman, is called sociobiology.

How did various types of societies develop? What selective advantage does a  member of a social group have? In what ways are social groups better adapted to their environment than nonsocial organisms? How does social organization affect the way populations grow and change? These are difficult questions because, although evolution occurs at the population level, it is individual organisms that are selected. Thus we need new ways of looking at evolution of social structures.

The ultimate step is to analyze human societies according to sociobiological principles. Such an analysis is difficult and controversial, however, because humans have a much greater ability to modify behavior than do other animals. However, when we look at human social behavior we see some clear parallels between human and nonhuman behaviors. This implies that there are certain fundamental similarities among social organisms regardless of their species. Do we see territorial behavior in our social species. Do groups of humans have dominance hierarchy? Most business, government, and social organizations have a clear dominance hierarchy in which those at the top get more resource (money, prestige) than those lower in the organization. Do human societies show division of labor? Our societies clearly benefit from the specialized skills of certain individuals. Do humans treat their own children differently from other children? Studies of child abuse indicate that abuse is more common between parents and their nongenetic stepchildren than between parents and their biological children. Although these few examples do not prove that human societies follow certain rules typical of other animal societies, it bears further investigation. Sociobiology will continue to explore the basis of social organization and behavior and will continue to be an interesting and controversial area of study.

A population is a group of organisms of the same species located in the same place at the same time. Examples are the number of dandelions in your front yard, the rat population in the sewers of your city, or the number of people of in your biology class. On a large scale, all the people of the world constitute the human population. The term species and population are interrelated because a species is a population—the largest possible population of a particular kind of organisms. The term population, however, is often used to refer to portions of a species by specifying a space and time. For example, the size of the human population in a city changes from hour to hour during the day and varies according to where you set the boundaries of the city.

Because each local population is a small portion of a species, we should expect distinct populations of the same species to show differences. One of the ways in which they can differ is in gene frequency. Chapter 11 on population genetics introduced you to the concept of gene frequency, which is a measure of how often a specific gene shows up in the gametes of a population. Two populations of the same species often have quite different gene frequencies. For example, many populations of mosquitoes have high frequencies of insecticide-resistant genes, whereas others do not. The frequency of the genes for tallness in human is greater in certain African tribe than in any other human population. Figure 16.1 shows that the frequency of the allele for type B blood differs significantly form one human population to another.

Because members of a population are of the same species, sexual reproduction can occur, and genes can flow from one generation to the next. Genes can also flow from one place to another as organisms migrate or are carried from one geographic location to another. Gene flow is used to refer to both the movement of genes within a species because of migration and the movement from one generation to the next as a result of gene replication and sexual reproduction. Typically both happen together as individuals migrate to new regions and subsequently reproduce, passing their genes to the next generation in the new area.

Another feature of a population is its age distribution, which is the number of organisms of each age in the population. In addition, organisms are often grouped into following categories:

1.        prereproductive juveniles—insect larvae, plant seedlings, or babies

2.        reproductive adults—mature insects, plants producing seeds, or humans in early adulthood

3.        postreproductive adults no longer capable of reproduction—annual plants that have shed their seeds, salmon that have spawned, and many elderly humans.

A population is not necessarily divided into equal thirds. In some situations, a population may be made up of a majority of one age group. If the majority of the population is prereproductive , than a “baby boom” should be anticipated in the future. If a majority of population is reproductive, the population should be growing rapidly, if the majority of the population is postreproductive, a population decline should be anticipated. Many organisms that live only a short time and have high reproductive rates can have age distributions that change significantly in a matter of weeks or months. For example, many birds have a flurry of reproductive activity during the summer months. Therefore, if you sample the population of a specific species of bird at different times during the summer you would have widely different proportions of reproductive and prereproductive individuals.

Populations can also differ in their sex ratios. The sex ratio is the number of males in a population compared to the number of females. In bird and mammal species where strong pair-bonding occurs, the sex ratio may be nearly one to one. Among mammals and birds that do not have strong pair-bonding, sex ratios may show a large number of females than males. This is particularly true among game species, where more males than females are shot, resulting in a higher proportion of surviving females. Because one male can fertilize several females, the population can remain large even though the females outnumber the males. However, if the population of these managed game species becomes large enough to cause problem, it becomes necessary to harvest some of the females as well because their number determines how much reproduction can take place. In addition to these examples, many species of animals like bison, horses, and elk have mating systems in which one male maintains a harem of females. The sex ratio in these small groups is quite different from a 1:1 ratio. There are very few situations in which the number of males exceeds the number of females. In some human and other populations, there may be sex ratios in which the males dominate if female mortality is unusually high or if some special mechanism separates most of one sex from the other.

Regardless of the sex ratio of a population, most species can generate large numbers of offspring, producing a concentration of organisms in an area. Population density is the number of organisms of a species per unit area. Some populations are extremely concentrated in a limited space; others are well dispersed. As the population density increases, competition among members of the population for the necessities of life increases. This increases the likelihood that some individuals will explore new habitats and migrate to new areas. Increases in the intensity of competition that causes changes in the environment and lead to dispersal are often referred to as population pressure. The dispersal of individuals to new areas can relieve the pressure on the home area and lead to the establishment of new populations. Among animals, it is often the juveniles who participate in this dispersal process. Female bears generally mate every other year and abandon their nearly grown young the summer before the next set of cubs is to be born. The abandoned young bears tend to wander and disperse to new areas. Similarly, young turtles, snakes, rabbits, and many other common animals disperse during certain times of the year. That is one of the reasons you see so many road-killed animals in the spring and fall.

If dispersal can not relieve population pressure, there is usually an increase in the rate at which individuals die because of predation, parasitism, starvation, and accidents. In plant populations, dispersal is not very useful for relieving population density; instead, the death of weaker individuals usually results in reduced population density. In the lodgepole pine, seedlings become established in areas following fire and dense thickets of young trees are established. As the stand ages, many small trees die and the remaining trees grow larger as the population density drops.

16.2 reproductive capacity

Sex ratio and age distributions within a population have a direct bearing on the rate of reproduction. Each species has an inherent reproductive capacity or biotic potential, which is the theoretical maximum rate of reproduction. Generally, this biotic potential is many times larger than the number of offspring needed simply to maintain the population. For example, a female carp may produce 1 million to 3 million eggs in her lifetime. This is her reproductive capacity. However, only two or three of these offspring ever develop into sexually mature adults. Therefore, her reproductive rate is much smaller than her reproductive potential.

     A high reproductive capacity is valuable to a species because it provides many slightly different individuals for the environment to select among. With most plants and animals, many of the potential gametes are never fertilized. An oyster may produce a million eggs a year, but not all of them are fertilized, and most that are fertilized die. An apple tree with thousands of flowers may produce only a few apples because the pollen that contains the sperm cells was not transferred to the female part of each flower in the process of pollination. Even after the new individuals are formed, mortality is usually high among the young. Most seeds that fall to the earth do not grow, and most young animals die. But, usually, enough survive to ensure continuance of the species. Organisms that reproduce in this way spend large amounts of energy on the production of gametes and young, without caring for the young. Thus the probability that any individual will reach reproductive age is small.

A second way of approaching reproduction is to produce relatively fewer individuals but provide care and protection that ensure a higher probability that the young will become reproductive adults. Humans generally produce a single offspring per pregnancy, but nearly all of them live. In effect, energy has been channeled into the care and protection of the young produced rather than into the production of incredible large amounts of potential young. Even though fewer young are produced by animals like birds and mammals, their reproductive capacity still greatly exceeds the number required to replace the parents when they die.

16.3 the population growth curve

Because most species of organisms have a high reproductive capacity, there is tendency for populations to grow if environmental conditions permit. For example, if the usual litter size for a pair of mice is 4, the 4 would produce 8, which in turn would produce 16, and so forth. Figure 16.5 shows a graph of change in population size over time known as a population growth curve. This kind of curve is typical for situations where a species is introduced into a previously unutilized area.

The change in the size of a population depends on the rate at which new organisms enter the population compared to the rate at which they leave. The number of new individuals added to the population by reproduction per thousand individuals is called natality. The number of individuals leaving the population by death per thousand individuals is called mortality. When a small number of organisms (two mice) first invade an area, there is a period of time before reproduction takes place during which the population remains small and relatively constant. This part of the population growth curve is known as the lag phase. During the lag phase both natality and mortality are low. The lag phase occurs because reproduction is not an instantaneous event. Even after animals enter an area they must mate and produce young. This may take days or years depending on the animal. Similarly, new plant introductions must grow to maturity, produce flowers, and set seed. Some annual plants may do this in less than a year, whereas some large trees may take several years of growth before they produce flowers.

In organisms that take a long time to mature and produce young, such as elephants, deer, and many kinds of plants, the lag phase may be measured in years. With the mice in example, it will be measured in weeks. The first litter of young will be able to reproduce in a matter of weeks. Furthermore，the original parents will probably produce an additional litter or two during this time period. Now we have several pairs of mice reproducing more than just once. With several pairs of mice reproducing, natality increases and mortality remains low; therefore the population begins to grow at an ever-increasing rate. This portion of the population growth curve is known as the exponential growth rate.

The number of mice (or any other organisms)) cannot continue to increase at a faster and faster rate because, eventually, something in the environment will become limiting and cause an increase in the number of deaths. For animals, food, water, or nesting sites may be in short supply, or predators or disease may kill many individuals. Plants may lack water, soil nutrients, or sunlight. Eventually, the number of individuals entering the population will come to equal the number of individuals leaving it by death or migration, and the population size becomes stable. Often there is both a decrease in natality and an increase in mortality at this point. This portion of the population growth curve is known as the stable equilibrium phase. It is important to recognize that this is still a population with births, deaths, migration, and a changing mix of individuals; however the size of the population is stable.

16.4 population-size limitation

Populations can not continue to increase indefinitely; eventually, some factor or set of factors acts to limit the size of a population, leading to the development of a stable equilibrium phase or even to a reduction in population size. The identifiable factors that prevent unlimited population growth are known as limiting factors. All the different limiting factors that act on a population are collectively known as environmental resistance, and the maximum population that an area can support is known as the carrying capacity of the area. In general, organisms that are small and have short life spans tend to have fluctuating populations and do not reach a carrying capacity, whereas large organisms that live a long time tend to reach an optimum population size that can be sustained over an extended period. A forest ecosystem contains populations of many insect species that fluctuate widely and rarely reach a carrying capacity, but the number of specific tree species or large animals such as owls or deer is relatively constant. Each is at the carrying capacity of the ecosystem for its species.

Carrying capacity is not an inflexible number, however. Often such environmental differences as successional changes, climate variations, disease epidemics, forest fires, or floods can change the carrying capacity of an area for specific species. In aquatic ecosystems one of the major factors that determine the carrying capacity is the amount of nutrients in the water. In areas where nutrients are abundant, the numbers of various kinds of organisms are high. Often nutrient levels fluctuate with changes in current or runoff from the land, and plant and animal populations fluctuate as well. In addition, a change that negatively affects the carrying capacity for one species may increase the carrying capacity for another. For example, the cutting down of a mature forest followed by the growth of young trees increases the carrying capacity for deer and rabbits, which use the new growth for food, but decreases the carrying capacity for squirrels, which need mature, fruit-producing trees as a source of food and old, hollow trees for shelter.

Wildlife management practices often encourage modification to the environment that will increase the carrying capacity for the designated game species. The goal of wildlife managers is to have the highest sustainable population available for harvest by hunters. Typical habitat modifications include creating water holes, cutting forests to provide young growth, and encouraging the building of artificial nesting sites.

In some cases the size of the organisms in a population also affects the carrying capacity. For example, an aquarium of a certain size can support only a limited number of fish, but the size of the fish makes a difference. If all the fish are tiny, a large number can be supported, and the carrying capacity is high; however, the same aquarium may be able to support only one large fish. In other words, the biomass of the population makes a difference. Similarly, when an area is planted with small trees, the population size is high. But as the trees get larger, competition for nutrients and sunlight becomes more intense, and the number of trees declines while the biomass increases.

16.5 categories of limiting factors

Limiting factors can be placed in four broad categories:

1.        availability of raw materials

2.        availability of energy

3.        production and disposal of waste products

4.        interaction with other organisms

The first category of limiting factors is the availability of raw materials. For example, plants require magnesium for the manufacture of chlorophyll, nitrogen for protein production, and water for the transport of materials and as a raw material for photosynthesis. If these substances are not present in the soil, the growth and reproduction of plants is inhibited. However, if fertilizer supplies these nutrients, or if irrigation is used to supply water, the effects of these limiting factors can be removed, and some other factor becomes limiting. For animals, the amount of water, minerals, materials for nesting, suitable burrow sites, or food may be limiting factors. Food for animals really fits into both this category and the next because it supplies both raw materials and energy.

The second major type of limiting factor is the availability of energy. The amount of light available is often a limiting factor as plants, which require light as an energy source for photosynthesis. Because all animals use other living things as sources of energy and raw materials, a major limiting factor for any animal is its food source.

The accumulation of waste productions is the third general category of limiting factors. It does not usually limit plant populations because they produce relatively few wastes. However, the buildup of high levels of self-generated waste products is a problem for bacterial populations and populations of tiny aquatic organisms. As wastes build up, they become more and more toxic, and eventually reproduction stops, or the population may even die out. When a few bacteria are introduced into a solution containing a source of food, they go through the kind of population growth curve typical of all organisms. As expected, the number of bacteria begins to increase following a lag phase, increases rapidly during the exponential growth phase, and eventually reaches stability in the stable equilibrium phase. But as waste products accumulate, the bacteria literally drown in their own wastes. When space for disposal is limited, and no other organisms are present that can convert the harmful wastes to less harmful products, a population decline known as the death phase follows.

Wine makers deal with this situation. When yeasts ferment the sugar in grape juice, they produce ethyl alcohol. When the alcohol concentration reaches a certain level, the yeast population stops growing and eventually declines. Therefore wine can naturally reach an alcohol concentration of only 12% to 15%. To make any drink stronger than that (of a higher alcohol content), water must be removed (to distill) or alcohol must be added (to fortify).

In small aquatic pools like aquariums, it is often difficult to keep populations of organisms healthy because of the buildup of ammonia in the water from the waste products of the animas. This is the primary reason that activated charcoal filters are commonly used in aquariums. The charcoal removes many kinds of toxic compounds and prevents the buildup of waste products.

The fourth set of limiting factors is organism interaction. As we learned in chapter 15 on community interaction, organisms influence each other in many ways. Some organisms are harmed and others benefit. The population size of any organism is negatively affected by parasitism, predation, or competition. Parasitism and predation usually involve interaction between two different species, although cannibalism of others of the same species does occur in some animals. Competition among members of the same species is often extremely intense. This is true for all kinds of organisms, not just animals.

On the other hand many kinds of organisms perform services for others that have beneficial effects on the population. For example, decomposer organisms destroy toxic waste products, thus benefiting populations of animals. They also recycle materials needed for the growth and development of all organisms. Mutualistic relationships benefit both populations involved. The absence of such beneficial organisms would be a limiting factor.

Often, the population sizes of two kinds of organisms are interdependent because each is a primary limiting factor of the other. This is most often seen in parasite-host relationships and predator-prey relationships. A good example is the relationship of the lynx (a predator) and the varying hare (the prey) as it was studied in Canada. The varying hare has a high reproductive capacity. In peak reproductive years, a female varying hare can produce 16 to 18 young. As with many animals a primary cause of death is predation. The varying hare population is a good food source for a variety of predators including lynx. When the population of varying hares increases it provides an abundant source of food for the lynx and the size of the lynx population rises, and when the population of hares decreases so does that of  the lynx. This pattern repeats itself in a ten-year cycle.

Recent studies indicate that one of the causes of the decline in varying hare populations is a reduction in their reproduction rate. The causes of this reduction rate may be related to a variety of factors including: reduced quality of food and higher levels of stress resulting from greater difficult in finding food and avoiding predators. With reduced reproduction rate and continued high predation the varying hare population drops. With reduced numbers of hares, lynx populations drop. Eventually the reproduction rate of hares increases and the population rebounds followed by a rebound in the lynx population as well. It appears that both food availability and predation are important limiting factors that determine the size of the varying hare population and the number of varying hares is a primary limiting factor for the lynx.

Extrinsic and intrinsic factors

Some factors that help control populations come from outside the population and are known as extrinsic factors. Predators, loss of food source, lack of sunlight, or accidents of nature are all extrinsic factors. However, many kinds of organisms self-regulate their population size. The mechanisms that allow them to do this are called intrinsic factors. For example, a study of rats under crowded living conditions showed that as conditions became more crowded, abnormal social behavior became common. There was a decrease in litter size, fewer litters per year were produced, mothers were more likely to ignore their young, and many young were killed by adults. Thus changes in the behavior of the members of the rat population itself resulted in lower birthrates and higher deathrates, leading to a reduction in the population growth rate. As another example, trees that are stressed by physical injury or disease often produce extremely large numbers of seeds (offspring) the following year. The trees themselves alter their reproductive rate. The opposite situation is found among populations of white-tailed deer. It is well known that reproductive success is reduced when the deer experience a series of severe winters. When times are bad, the female deer are more likely to have single offspring rather than twins.

Density-dependent and density-independent limiting factors

Many populations are controlled by limiting factors that become more effective as the size of the population increases. Such factors are referred to as density-dependent factors. Many of the factors we have already discussed are density-dependent. For example, the larger a population becomes, the more likely it is that predators will have a chance to catch some of the individuals. A prolonged period of increasing population allows the size of the predator population to increase as well. Large populations with high population density are more likely to be affected by epidemics of parasites than are small populations of widely dispersed individuals because dense populations allow for the easy spread of parasites from one individual to another. The rat example discussed previously is another good example of a density-dependent factor operating because the amount of abnormal behavior increased as the density of the population increased. In general, whenever there is competition among members of a population, its intensity increases as the population increases. Large organisms that tend to live a long time and have relatively fewer young are most likely to be controlled by density-dependent factors.

Density-independent factors are population-controlling influences that are not related to the size of the population. They are usually accidental or occasional extrinsic factors in nature that happen regardless of the size or density of a population. A sudden rainstorm may drown many small plant seedlings and soil organisms. Many plants and animals are killed by frosts that come late in spring or early in the fall. A small pond may dry up, resulting in the death of many organisms. The organisms most likely to be controlled by density-independent factors are small, short-lived organisms that can reproduce very rapidly.

So far we have looked at populations primarily from a nonhuman point of view. Now it is time to focus on the human species and the current problem of world population growth.

16.6 limiting factors to human population growth

Today we hear differing opinions about the state of the world’s human population. On one hand we hear that the population is growing rapidly. By contrast we hear that some countries are afraid that their populations are shrinking. Other countries are concerned about the aging of their populations because birthrates and deathrates are low. In magazines and on television we see that there are starving people in the world. At the same time we hear discussions about the problem of food surpluses and obesity in many countries. Some have even said that the most important problem in the world today is the rate at which the human population is growing; others maintains that the growing population will provide markets for goods and be an economic boon. How do we reconcile this mass of conflicting information?

It is important to realize that human populations follow the same patterns of growth and are acted upon by the same kinds of limiting factors as are population of other organisms. When we look at the curve of population growth over the past several thousand years, estimates are that the human population remained low and constant for thousands of years but has increased rapidly in the past few hundred years. For example, it has been estimated that when Columbus discovered American, the Native American population was about 1 million and was at or near its carrying capacity. Today, the population of North American is over 300 million people. Does this mean that humans are different from other animal species? Can the human population continue to grow forever?

The human species is no different from other animals. It has an upper limit set by the carrying capacity of the environment but the human population has been able to increase astronomically because technological changes and displacement of other species has allowed us to shift the carrying capacity upward. Much of the exponential growth phase of the human population can be attributed to the removal of diseases, improvement in agricultural methods, and replacement of natural ecosystems with artificial agricultural ecosystems. But even these conditions have their limits. There will be some limiting factors that eventually cause a leveling off of our population growth curve. We can not increase beyond our ability to get raw materials and energy, nor can we ignore the waste products we produce or the other organisms with which we interact.

Available raw materials

To many of us, raw materials consists simply the amount of food available, but we should not forget that in a technological society, iron ore, lumber, irrigation water, and silicon chips are also raw materials. However, most people of the world have much more basic needs. For the past several decades, large portion of the world’s population have not had enough food. Although it is biologically accurate to say that the world can currently produce enough food to feed all the people of the world, there are many reasons why people can’t get food or won’t eat it. Many cultures have food taboos or traditions that prevent the use of some available food sources. For example, pork is forbidden in some cultures. Certain groups of people find it almost impossible to digest milk. Some Africa cultures use a mixture of cow’s milk and cow’s blood as food, which people of other cultures might be unable to eat.

In addition, there are complex political, economic, and social issues related to the production and distribution of food. In some cultures, farming is a low-status job, which means that people would rather buy their food from someone else than grow it themselves. This can result in underutilization of agricultural resources. Food is sometimes used as a political weapon when governments want to control certain groups of people. But probably most important is the fact that transportation of food from centers of excess to centers of need is often difficult and expensive.

A more fundamental question is whether the world can continue to produce enough food. In 2001 the world population was growing at a rate of 1.3% per year. This amounts to about 160 new people added to the world population every minute, which will result in a doubling of the world population in about 50 years. With a continuing increase in the number of mouths to feed, it is unlikely that food production will be able to keep pace with the growth in human population. A primary indicator of the status of the world food situation is the amount of grain produced for each person in the world (per capita grain production). World per capita grain production peaked in 1984. the less-developed nations of the world have a disproportionately large increase in population and a decline in grain production because they are less able to afford costly fertilizer, machinery, and the energy necessary to run the machines and irrigate the land to produce their own grain.

Availability of energy

The availability of energy is the second broad limiting factor that affects human populations as well as other kinds of organisms. All species on Earth ultimately depend on sunlight for energy—including the human species. Whether one produces electrical power from a hydroelectric dam, burns fossil fuels, or uses a solar cell, the energy is derived from the Sun. Energy is needed for transportation, building and maintaining homes, and food production. It is very difficult to develop unbiased, reasonably accurate estimates of global energy “reserves” in the form of petroleum, natural gas, and coal. Therefore, it is difficult to predict how long these “reserves” might last. We do know, however, that the quantities are limited and that the rate of use has been increasing.

If the less-developed countries were to attain a standard living equal to that of the developed nations, the global energy “reserves” would disappear overnight. Because the United States constitutes approximately 4.6% of the world’s population and consumes approximately 25% of the world’s energy resources, raising the standard of living of the entire world population to that of the United States would result in a tremendous increase in the rate of consumption of the energy and reduce theoretical reserves dramatically. Human would realize that there is a limit to our energy resources; we are living on solar energy that was stored over millions of years, and we are using it at a rate that could deplete it in hundreds of years. Will energy availability be the limiting factor that determines the ultimate carrying capacity for humans, or will problems of waste disposal predominate?

Production of wastes

One of the most talked-about aspects of human activity is the problem of waste disposal. Not only do we have normal biological wastes, which can be dealt with by decomposer organisms, but we generate a variety of technological wastes and by-products that can not be efficiently degraded by decomposers. Most of what we call pollution results from the waste products of technology. The biological wastes usually can be dealt with fairly efficiently by building waste water treatment plants and other sewage facilities. Certainly these facilities take energy to run, but they rely on decomposers to degrade unwanted organic matter to carbon dioxide and water. Earlier in this chapter we discussed the problem that bacteria and yeast face when their bolic waste products accumulate. In this situation, the organisms so “befoul their nest” that their waste poison themselves. Are humans in a similar situation on a much larger scale? Are we dumping so much technological waste, much of which is toxic, into the environment that we are being poisoned? Some people believe that disregard for the quantity of our environment will be a major factor in decreasing our population growth rate. In any case, it makes good sense to do everything possible to stop pollution and work toward cleaning out nest.

Interaction with other organisms

The fourth category of limiting factors that determine carrying capacity is interaction among organisms. Humans interaction with other organisms in as many ways as other animals do. We have parasites and occasionally predators. We are predators in relation to a variety of animals, both domesticated and wild. We have mutualistic relationships with many of our domesticated plants and animals because they could not survive without our agricultural practices and we would not survive without the food they provide. Competition is also very important. Insects and rodents compete for the food we raise, and we compete directly with many kinds of animals for the use of ecosystems.

As humans convert more and more land to agriculture and other purposes, many other organisms are displaced. Many of these displaced organisms are not able to compete successfully and must leave the area, have their population reduced, or become extinct. The American bison (buffalo), African and Asian elephants, panda, and grizzly bear are a few species that are considerably reduced in number because they were not able to compete successfully with the human species. The passenger pigeon, Carolina parakeet, and great auk are a few that have become extinct. Out parks and natural areas have become tiny refuges for plants and animals that once occupied vast expanses of the world. If these refuges are lost, many organisms will become extinct. What today might seem to be an insignificant organism that we can easily do without may tomorrow be seen as a link to our very survival. We humans have been extremely successful in our efforts to convert ecosystems to our own uses at the expense of other species.

Competition with one another (intraspecific competition), however, is a different matter. Because competition is negative to both organisms, competition between humans harms humans. We are not displacing another species, we are displacing some of our own kind. Certainly, when resources are in short supply, there is competition. Unfortunately, it is usually the young that are least able to compete, and high infant mortality is the result.

Control of human population is a social problem

Humans are different from most other organisms in a fundamental way: we are able to predict the outcome of a specific course of action. Current technology and medical knowledge are available to control human population and improve the health and well-being of the people of the world. Why then does human population continue to grow, resulting in human suffering and stressing the environment in which we live? Because we are social animals that have freedom of choice, we frequently do not do what is considered “best” from an unemotional, unselfish, biological point of view. People make decisions based on historical, social, cultural, ethical, and personal considerations. What is best for the population as a whole may be bad for you as an individual.

The biggest problems associated with control of the human population are not biological problems but, rather, require the efforts of philosophers, theologians, politicians, and sociologists. As population increase, so will political, social, and biological problems; individual freedom will diminish, intense competition for resource will intensify, and famine and starvation will become even more common. The knowledge and technology necessary to control the human population are available, but the will is not. What will eventually limit the size of our population? Will it be lack of resources, lack of energy, accumulated waste products, competition among ourselves, or rational planning of family size?

Recent studies of the changes in the population growth rates of different countries indicate that a major factor determining the size of families is the educational status of women. Regardless of other cultural differences, as girls and women become educated they have fewer children. Several reasons have been suggested for this trend, higher levels of education allow women to get jobs with higher pay, which makes them less dependent on males for their support. Being able to read may lead to better comprehension of how methods of birth control work. Regardless of the specific reasons, improving educational levels of women has now become a major technique employed by rapidly growing countries that hope to eventually control their populations.

People approach the study of organism interactions in two major ways. Many people look at interrelationships from the broad ecosystem point of view; others focus on individual organisms and the specific things that affect them inn their daily lives. The first approach involves the study of all the organisms that interact with one another—the community—and usually looks at general relationships among them. Chapter 14 described categories of organisms—producers, consumers, and decomposers—that perform different s in a community.

Another way of looking at interrelationships is to study in detail the ecological relationships of certain species of organisms. Each organism has particular requirements for life and lives where the environment provides what it needs. The environmental requirements of a whale include large expanses of ocean, but with seasonally important feeding areas and protected locations used for giving birth. The kind of place, or part of an ecosystem, occupied by an organism is known as its habitat. Habitats are usually described in terms of conspicuous or particularly significant features in the area where the organism lives. For example, the habitat of a prairie dog is usually described as a grassland and the habitat of a tuna is described as the open ocean. The habitat of the fiddler crab is sandy ocean shores and the habitat of various kinds of cacti is the desert. The key thing to keep in mind when you think of habitat is the place in which a particular kind of organism lives. In our deions of the habitats of organisms, we sometimes use the terminology of the major biomes of the world, such as desert, grassland, or savanna, but it is also possible to describe the habitat of the bacterium Escherichia coli as the gut of humans and other mammals, or the habitat of a fungus as a rotting log. Organisms that have very specific places in which they live simply have more restricted habitats.

Each species has particular requirements for life and places specific demands on the habitat in which it lives. The specific al role of an organism is its niche. Its niche is the way it goes about living its life. Just as the word place is the key to understand the concept of habitat, the word is the key to understand the concept of a niche. To understand the niche of an organism involves a detailed understanding of the impacts an organism has on its biotic and abiotic surroundings as well as all the factors that affect the organism. For example, the niche of an earthworm includes abiotic items such as soil particle size; soil texture; and the moisture, pH, and the temperature of the soil. The eathworm’s niche also includes biotic impacts such as serving as food for birds, moles, and shrews; as bait for anglers, or as a consumer of dead plant organic matter. In addition, an earthworm serves as a host for variety of parasites, transports minerals and nutrients from deeper soil layers to the surface, incorporates organic matter into the soil, and creates burrows that allow air and water to penetrate the soil more easily. And this is only a limited sample of all the aspects of its niche.

Some organisms have rather broad niches; others, with very specialized requirements and limited roles to play, have niches that are quite narrow. The opossum is an animal with a very broad niche. It eats a wide variety of plant and animal foods, can adjust to a wide variety of climates, is used as food by many kinds of carnivores (include humans), and produces large numbers of offspring. By contrast, the koala of Australia has a very narrow niche. It can live only in areas of Australia with specific species of eucalyptus trees because it eats the leaves of only a few kinds of these trees. Furthermore, it cannot tolerate low temperatures and does not produce large numbers of offspring. As you might guess, the opossum is expanding its range, and the koala is endangered in much of its range.

The complete deion of an organisms’ niche involves a very detailed inventory of influences, activities, and impacts. It involves what the organism does and what is done to the organism. Some of the impacts are biotic, others are abiotic. Because the niche of an organism is a complex set of items, it is often easy to overlook important roles played by some organisms.

For example, when Europeans introduced cattle into Australia—a continent where there had previously been no large, hoofed mammals—they did not think about the impact of cow manure or the significance of a group of beetles called dung beetles. These beetles rapidly colonize fresh dung and cause it to be broken down. No such beetles existed in Australia; therefore, in areas where cattle were raised, a significant amount of land became covered with accumulated cow dung. This produced the area where grass could grow and reduced productivity. The problem was eventually solved by the importation of several species of dung beetles from Africa, where large, hoofed mammals are common. The dung beetles made use of what the cattle did not digest, returning it to a form that plants could more easily recycle into plant biomass.

15.2 kinds of organism interactions

One of the important components of an organism’s niche is the other living things with which it interacts. When organisms encounter one another in their habitats, they can influence one another in numerous ways. Some interactions are harmful to one or both of the organisms. Others are beneficial. Ecologists have classified kinds of interactions between organisms into several broad categories, which we will discuss here.

Predation

Predation occurs when one animal captures, kills, and eats another animal. The organism that is killed is called the prey, ant the one that does the killing is called the predator. The predator obviously benefits from the relationship; the prey organism is harmed. Most predators are relatively large compared to their prey and have specific adaptations that aid them in catching prey. Many spiders build webs that serve as nets to catch flying insects. The prey are quickly paralyzed by the spider’s bite and wrapped in a tangle of silk threads. Other rapidly moving spiders, like wolf spiders and jumping spiders, have large eyes that help them find prey without using webs. Dragonflies patrol areas where they can capture flying insects. Hawks and owls have excellent eyesight that allows them to find their prey. Many predators, like leopards, lions, and cheetahs, use speed to run down their prey; others such as frogs, toads, and many kinds of lizards blend in with their surroundings and strike quickly when a prey organism happens by.

    Many kinds of predators are useful to us because they control the populations of organisms that do us harm. For example, snakes eat many kinds of rodents that eat stored grain and other agricultural products. Many birds and bats eat insects that are agricultural pests. It is even possible to think of a predator as having a beneficial effect on the prey species. Certainly the individual organism that is killed is harmed, but the population can benefit. Predators can prevent large populations of prey organisms from destroying their habitat by hindering overpopulation of prey species of they can reduce the likelihood of epidemic disease by eating sick or diseased individuals. Furthermore, predators act as selecting agents. The individuals who fall to them as prey are likely to be less well adapted than the ones that escape predation. Predators usually kill slow, unwary, sick, or injured individuals. Thus the genes that may have contributed to slowness, inattention, illness, or the likelihood of being injured are removed from the gene pool and a better-adapted population remains. Because predators eliminate poorly adapted individuals, the species benefits. What is bad for the individual can be good for the species.

Parasitism

Another kind of interaction in which one organism is harmed and the other aided is the relationship of parasitism. In fact, there are more species of parasites in the world than there are nonparasites, making this a very common kind of relationship. Parasitism involves one organism living in or on another living organism from which it derives nourishment.

The parasite derives the benefit and harms the host, the organism it lives in or on. Many kinds of fungi live on trees and other kinds of plants, including those that are commercially valuable. Dutch elm disease is caused by a fungus that infects the living, sap-carrying parts of the tree. Mistletoe is a common plant that is a parasite on other plants. The mistletoe plant invades the tissues of the tree it is living on and derives nourishment from the tree.

Many kinds of worms, protozoa, bacteria, and viruses are important parasites. Parasites that live on the outside of their host are called external parasites. For example, fleas live on the outside of the bodies of mammals like rats, dogs, cats, and humans, where they such blood and do harm to their hosts. At the same time, the host could also have a tapeworm in its intestine. Because the tapeworm lives inside the host, it is called an internal parasite. Another kind of parasite that may be found in the blood of rats is the bacterium Yersinia pestis. It does little harm to the rat but causes disease known as Plague or Black Death if it is transmitted to humans. Because fleas can such the blood of rats and also live on and bite humans they can serve as carries of bacteria between rats and humans. An organism that can carry a disease from one individual to another is called a vector. During the mid-1300s, when living conditions were poor and rats and fleas are common, epidemics of plague killed millions of people. In some countries in western Europe, 50% of the population was killed by this disease. Plague is still a problem today when living conditions are poor and sanitation is lacking. Cases of plague are even found in developed countries like the United States on occasion.

Lyme disease is also a vector-borne disease caused by the bacterium, Borrela burgdorferi, which is spread by certain species of ticks. Over 90% of cases are centered in the Northeast.

Both predation and parasitism are relationships in which one member of the pair is helped and the other is harmed. But there are many kinds of interactions in which one is harmed and the other aided that don’t fit neatly into the categories of interactions dreamed up by scientists. For example, when a cow eats grass, it is certainly harming the grass while deriving benefit from it. We could call cows grass predators, but we usually refer to them as herbivores. Likewise, such animals as mosquitoes, biting flies, vampire bats, and ticks take blood meals but don’t usually permanently on the host or kill it. Are they temporary parasites or specialized predators? Finally, birds like cowbirds and some species of European cuckoos lay their eggs in the nests of other species of birds, who raise these foster young rather than their own. The adult cowbird and cuckoo often remove eggs from the host nest or their offspring eject the eggs or the young of the host-bird species, so that usually only the cowbird or cuckoo is raised by the foster parents. This kind of relationship has been called nest parasitism, because the host parent birds are not killed and aid the cowbird or cuckoo by raising their young.

commensalism

both predation and parasitism involve one organism benefiting while the other is harmed. Another common relationship is one in which one organism benefits and the other is not affected. This is known as commensalisms. For example, sharks often have another fish, the remora, attached to them. The remora has a sucker on the top side of its head that allows it to attach to the shark and get a free ride. Although the remora benefits from the free ride and by eating leftovers from the shark’s meals, the shark does not appear to be troubled by this uninvited guest, nor does it benefit from the presence of the remora.

Another example of commersalism is the relationship between trees and epiphytic plants. Epiphytes are plants that live on the surface of other plants but do not derive nourishment from them. Many kinds of plants (e.g., orchids, fern, and mosses) use the surfaces of trees as places to live. These kinds of organisms are particularly common in tropical rainforests. Many epiphytes derive benefit from the relationship because they are able to be located in the tops of the trees, where they receive more sunlight and moisture. The tree derives no benefit from the relationship, nor are they harmed; they serve as support surfaces for epiphytes.

mutualism

so far in our examples, only one species has benefited from the association of two species. There are also many situations in which two species live in close association with one another, and both benefit. This is called mutualism. One interesting example of mutualism involves digestion in rabbits. Rabbits eat plant material that is high in cellulose even though they do not produce the enzymes capable of breaking down cellulose molecules into simple sugars. They manage to get energy out of these cellulose molecules with the help of special bacteria living in their digestive tracts. The bacteria produce cellulose-digesting enzyme, called cellulases, that break down cellulose into smaller carbohydrate molecules that the rabbit’s digestive enzymes can break down into smaller glucose molecules. The bacteria benefit because the gut of the rabbit provides them with a moist, warm, nourishing environment in which to live. The rabbit benefits because the bacteria provide them with a source of food. Termites, cattle, buffalo, and antelope also have collections of bacteria and protozoa living in their digestive tracts that help them digest cellulose.

Another kind of mutualistic relationship exists between flowering plants and bees. Undoubtedly you have observed bees and other insects visiting flowers to obtain nectar from the blossoms. Usually the flowers are constructed in such a manner that the bees pick up pollen (sperm-containing packages) on their hairy bodies, which they transfer to the female part of the next flower they visit. Because bees normally visit many individual flowers of the same species for several minutes and ignore other species of flowers, they can serve as pollen carriers between two flowers of the same species. Plants pollinated in this manner produce less pollen than do plants that rely on the wind to transfer pollen. This saves the plant energy because it doesn’t need to produce huge quantities of pollen. It does, however, need to transfer some of its energy savings into the production of showy flowers and nectar to attract the bees. The bees benefit from both the nectar and pollen; they use both for food.

Lichens and corals exhibit a more intimate kind of mutualism. In both cases the organisms consist of the cells of two different organisms intermingled with one another. Lichens consist of fungal cells and algal cells in a partnership; corals consist of the cells of the coral organism intermingled with algal cells. In both cases, the algae carry on photosynthesis and provide nutrients and fungus or coral provides a moist, fixed structure for the algae to live in.

One other term that relates to parasitism, commensalisms, and mutualism is symbiosis. Symbiosis literally means “living together”. Unfortunately, this word is used in several ways, none of which is very precise. It is often used as a synonym for mutualism, but it is also often used to refer to commensal relationships and parasitism. the emphasis, however, is on interactions that involve a close physical relationship between the two kinds of organisms.

Competition

So far in our discussion of organism interactions we have left out the most common one. It is reasonable to envision every organism on the face of the earth being involved in competitive interactions. Competition is a kind of interaction between organisms in which both organisms are harmed to some extent. Competition occurs whenever two organisms need a vital resource that is in short supply. The vital resource could be food, shelter, nesting sites, water, mates, or space. It can be a snarling tug-of-war between two dogs over a scrap of food, or it can be a silent struggle between plants for access to available light. If you have ever started tomato seeds (or other garden plants) in a garden and failed to eliminate the weeds, you have witnessed competition. If the weeds are not removed, they compete with the garden plants for available sunlight, water, and nutrients, resulting in poor growth of both the garden plants and the weeds.

The more similar the requirements of the two species of organisms, the more intense the competition is. According the competitive exclusion principle, no two species of organisms can occupy the same niche at the same time. If two species of organisms do occupy the same niche, the competition will be so intense that one or more of the following will occur: one will become extinct, one will be forced to migrate to a different area, or the two species may evolve into slight different niches so that they do not compete.

It is important to recognize that although competition results in harm to both organisms there can still be winners and losers. The two organisms may not be harmed to the same extent with the result that one will have greater access to the limited resource. Furthermore, even the loser can continue to survive if it migrates to an area where competition is less intense or evolves to exploit a different niche. Thus competition provides a major mechanism for natural selection. With the development of slight differences between niches the intensity of competition is reduced. For example, many birds catch flying insects as food. However, they do not compete directly with each other because some feed at night, some feed high in the air, some feed only near the ground, and still others perch on branches and wait for insect to fly past. The insect-eating niche can be further subdivided by specialization on particular sizes of kinds of insects.

Many of the relationships just described involve the transfer of nutrients from one organism to another (predation, parasitism, mutualism). Another important way scientists look at ecosystems is to look at how materials are cycled from organism to organism.

15.3 the cycling of materials in ecosystems

Although some new atoms are being added to the Earth from cosmic dust and meteorites, this amount is not significant in relation to the entire biomass of the Earth. Therefore, the earth can be considered to be a closed ecosystem as far as matter is concerned. Only sunlight energy comes to the earth in a continuous stream, and even this is ultimately returned to space as heat energy. However, it is this flow of energy that drives all biological processes. Living systems have evolved ways of using this energy to continue life through growth and reproduction and the continual reuse of existing atoms. In this recycling process, inorganic molecules are combined to form the organic compounds of living things. If there were no way of recycling this organic matter back into its inorganic forms, organic material would build up as the bodies of dead organisms. This is though to have occurred millions of years ago when the present deposits of coal, oil, and natural gas were formed. Under most conditions decomposers are available to break down organic material to inorganic material that then can be reused by other organisms to rebuild organic material. One way to get an appreciation of how various kinds of organisms interact to cycle materials is to look at a specific kind of atom and follow its progress through an ecosystem

The carbon cycle

Living systems contain many kinds of atoms, but some are more common than others, carbon, nitrogen, oxygen, hydrogen, and phosphorus are found in all living things and must be recycled when an organism dies. Let’s look at some examples of this recycling process. Carbon and oxygen atoms combine to form the molecule carbon dioxide, which is a gas found in small quantities in the atmosphere. During photosynthesis, carbon dioxide combines with water to form complex organic molecules like sugar. At the same time, oxygen molecules are released into the atmosphere.

The organic matter in the bodies of plants may be used by herbivores as food. When an herbivore eats a plant, it breaks down the complex organic molecules into more simple molecules, like simple sugars, amino acids, glycerol, and fatty acids. These can be used as building blocks in the construction of its own body. Thus the atoms in the body of the herbivore can be traced back to the plants that were eaten. Similarly, when herbivores are eaten by carnivores, these same atoms are transferred to them. Finally, the waste products of plants and animals and the remains of dead organisms are used by decomposer organisms as sources of carbon and other atoms they need for survival. In addition, all the organisms in this cycle—plants, herbivores, carnivores, and decomposers—obtain energy (ATP, adenosine triphosphate) from the process of respiration, in which oxygen is used to break down organic compounds into carbon dioxide and water. Thus the carbon atoms that started out as components of carbon dioxide molecules have passed through the bodies of living organisms as parts of organic molecules and returned to the atmosphere as carbon dioxide, ready to be cycled again. Similarly, the oxygen atoms released as oxygen molecules during photosynthesis have been used during the process of respiration.

The hydrogen cycle

Water molecules are the most common molecules in living things and are essential for life. Water molecules are used as raw materials in the process of photosynthesis. The hydrogen atom from water molecules are added to carbon atoms to make carbohydrates and other organic molecules. Furthermore, the oxygen atoms in water molecules are released during photosynthesis as oxygen molecules. In addition, all the bolic reactions that occur in organisms take place in a watery environment. We can trace the movement and reuse of water molecules by picturing a hydrologic cycle.

Most of the forces that cause water to be cycled do not involve organisms, but are the result of normal physical process. Because of the kinetic energy possessed by water molecules, at normal Earth temperatures liquid water evaporates into the atmosphere as water vapor. This can occur wherever water is present; it evaporates from lakes, rivers, soil, or the surfaces of organisms. Because the oceans contain most of the world’s water, an extremely large amount of water enters the atmosphere from the oceans. In addition, transpiration in plants involves the transport of water from the soil to leaves, where it evaporates. The movement of water carries nutrients to the leaves and the evaporation of water assists in the movement of water upward in the stem.

Once the water molecules are in the atmosphere, they are moved by prevailing wind patterns. If warm, moist air encounters cooler temperatures, which often happens over landmasses, the water vapor condenses into droplets and falls as rain or snow. When the precipitation falls on land, some of it runs off the surface, some of it evaporates, and some penetrates into the soil. The water in the soil may be taken up by plants and transpired into the atmosphere, or it may become groundwater. Much of the groundwater also makes its way into lakes and streams and ultimately arrives at the ocean from which it originated.

The nitrogen cycle

Another important element for living things is nitrogen. Nitrogen is essential in the formation of amino acids, which are needed to form proteins, and in the formation of nitrogenous bases, which are a part of ATP and the nucleic acids DNA and RNA. Nitrogen is found as molecules of nitrogen gas in the atmosphere. Although nitrogen gas makes up approximately 80% of the Earth’s atmosphere, only a few kinds of bacteria are able to convert it into nitrogen compounds that other organisms can use. Therefore, in most terrestrial ecosystems, the amount of nitrogen available limits the amount of plant biomass that can be produced. (most aquatic ecosystems are limited by the amount of phosphorus rather than the amount of nitrogen.) plants utilize several different nitrogen-containing compounds to obtain the nitrogen atoms they need to make amino acids and other compounds.

Symbiotic nitrogen-fixing bacteria live in the roots of certain kinds of plants, where they convert nitrogen gas molecules into compounds that the plants can use to make amino acids and nucleic acids. The most common plants that enter this mutualistic relationship with bacteria are legumes such as beans, clover, peas, alfalfa, and locust trees. Some other organisms, such as alder trees and even a kind of aquatic fern can also participate in this relationship. There are also free-living nitrogen-fixing bacteria in the soil that provide nitrogen compounds that can be take up through the roots, but the bacteria do not live in a close physical union with plants.

Another way plants get usable nitrogen compounds involves a series of different bacteria. Decomposer bacteria convert organic nitrogen-containing compounds into ammonia. Nitrifying bacteria can convert ammonia into nitrite-containing compounds, which in turn can be converted into nitrate-containing compounds. Many kinds of plants can use either ammonia or nitrate from the soil as building blocks for amino acids and nucleic acids.

All animals obtain their nitrogen from the food they eat. The ingested proteins are broken down into their component amino acids during digestion. These amino acids can then be reassembled into new proteins characteristic of the animal. All dead organic matter and waste products of plants and animals are acted upon by decomposer organisms, and the nitrogen is released as ammonia, which can be taken up by plants or acted upon by nitrifying bacteria to make nitrate.

Finally, other kinds of bacteria called denitrifying bacteria are capable of converting nitrite to nitrogen gas, which is released into the atmosphere. Thus, in the nitrogen cycle, nitrogen from the atmosphere is passed through a series of organisms, many of which are bacteria, and ultimately returns to the atmosphere to be cycled again. However, there is also a secondary cycle in which nitrogen compounds are recycled without returning to the atmosphere.

Because nitrogen is in short supply in most ecosystems, farmers usually find it necessary to supplement the natural nitrogen sources in the soil to obtain maximum plant growth. This can be done in a number of ways. Alternating nitrogen-producing crops with nitrogen-demanding crops helps maintain high levels of usable nitrogen in the soil. One year a crop such as beans or clover that has symbiotic nitrogen-fixing bacteria in its roots can be planted. The following year the farmer can plant a nitrogen-demanding crop such as corn. The use of manure is another way of improving nitrogen levels. The waste products of animals are broken down by decomposer bacteria and nitrifying bacteria, resulting in enhanced levels of ammonia and nitrate. Finally, the farmer can use industrially produced fertilizers containing ammonia or nitrate. These compounds can be used directly by plants or converted into other useful forms by nitrifying bacteria.

The phosphorus cycle

Phosphorus is another kind of atom common in the structure of living things. It is present in many important biological molecules such as DNA and in the membrane structure of cells. In addition, the bones and teeth of animals contain significant quantities of phosphorus. The ultimate source of phosphorus atoms is rock. In nature, new phosphorus compound are released by the erosion of rock and dissolving in water. Plants use the dissolved phosphorus compounds to construct the molecules they need. Animals obtain the phosphorus they need when they consume plants or other animals. When an organism dies or excretes waste products, decomposer organisms recycle the phosphorus compounds back into the soil. Phosphorus compounds that are dissolved in water are ultimately precipitated as deposits. Geologic elevate these deposits and expose them to erosion, thus making these deposits available to organisms. Waste products of animals often have significant amounts of phosphorus. In places where large numbers of seabirds or bats congregate for hundreds of years, the thickness of their droppings (called guano) can be a significant source of phosphorus for fertilizer. In many soils, phosphorus is in short supply and must be provided to crops plants to get maximum yields. Phosphorus is also in short supply in aquatic ecosystems.

Fertilizers usually contain nitrogen, phosphorus, and potassium compounds. The numbers on a fertilizer bag indicate the percentage of each in the fertilizer. For example, a 6-24-24 fertilizer has 6% nitrogen, 24% phosphorus, and 24% potassium compounds. In addition to carbon, nitrogen, and phosphorus, potassium and other elements are cycled within ecosystems. In an agriculture ecosystem, these elements are removed when the crop is harvested. Therefore farmers must not only return the nitrogen, phosphorus, and potassium, but they must also analyze for other less prominent elements and add them to their fertilizer mixture as well. Aquatic ecosystems are also sensitive to nutrient levels. High levels of nitrates or phosphorus compounds often result in rapid growth of aquatic producers. In aquaculture, such as that used to raise catfish, fertilizer is added to the body of water to stimulate the production of algae which is the base of many aquatic food chains.

15.4 the impact of human actions on communities

As you can see from this discussion and from the discussion of food webs in chapter 14, all organisms are associated in a complex network of relationships. A community consists of all these sets of interrelations. Therefore, before one decides to change a community, it is wise to analyze how the organisms are interrelated. This is not always an easy task because there is much we still do not know about how organisms interact and how they utilize molecules from their environment. Several lessons can be learned from studying the effects of human activity on communities.

Introduce species

One of the most far-reaching effects humans have bad on natural ecosystems involves the introduction of foreign species. Most of these introductions have been conscious decisions. Nearly all of our domesticated plants and animals are introductions from elsewhere. Cattle, horses, pigs, goats, and many introduced grasses have significantly altered the original ecosystems present in the Americas. Nearly all of our agriculturally important plants and animals are not native to North America. (corn, beans, sunflowers, squash, and the turkey are exceptions.) pigs have become a major problem in Hawaii and many other places in the world where they destroy the natural ecosystem by digging up roots and preventing the reproduction of native plants. The introduction of grasses as food for cattle has resulted in the decline of many native species of grasses and other plants that were originally part of grassland ecosystems. In Australia the introduction of domesticated plants and animals, and wild animals, such as rabbits and foxes, has severely reduced the populations of many native marsupial mammals.

Accidental introductions have also significantly altered ecosystems. Chestnut blight essentially eliminated the American chestnut from the forests of eastern North America. Similarly a fungal disease (Dutch elm disease) has severely reduced the number of elms in forests.

Predator control

During the formative years of wildlife management, it was though that the populations of game species could be increased if the populations of their predators were reduced. Consequently, many states passed laws that encouraged the killing of foxes, eagles, hawks, owls, coyotes, cougars, and other predators that use game animals as a source of food. Often bounties were paid to people who killed these predators. In south Dakota it was decided to increase the pheasant population by reducing the numbers of foxes and coyotes. However, when the supposed predator populations were significantly reduced, there was no increase in the pheasant population. There was rapid increase in the rabbit and mouse populations, however, and they became serious pests. Evidently the foxes and coyotes were major factors in keeping rabbit and mouse populations under control but had a minor impact on pheasants.

The absence of predators can lead to many kinds of problems with prey species. In many metropolitan areas deer have become pests. This is due to several reasons, including the fact that there are no predators, and hunting (predation by humans) is either not allowed or is impractical because of the highly urbanized nature of the area. Some municipalities have instituted programs of chemical birth control for their deer populations. In parts of Florida increased numbers of alligators because humans are the only effective predators of large alligators. Only a few years ago the alligator was on the endangered species list and all hunting was suspended. Similarly, in Yellowstone National Park, elk, bison, and moose populations have become very large because hunting is not allowed and predators are in low numbers. In 1995 wolves were reintroduced to the park in the hope that they would help bring the elk and moose populations under control. This was a controversial decision because ranchers in the vicinity do not want a return of large predators that might prey on their livestock. They are also opposed to having bison, many of which carry a disease that can affect cattle, stray onto their land. The wolf populations have increased significantly in Yellowstone and are having an effect on the populations of bison, elk, and moose. Regardless of the politics involved in the decision, Yellowstone is in a more natural condition today with wolves present than it was prior to 1995.

By contrast, the state of Alaska instituted a project to kill wolves because they believe the wolves are reducing caribou populations below optimal levels. Caribou hunting is an important source of food for Alaska natives, and hunters who visit the state provide a significant source of income. Many groups oppose the killing of wolves in Alaska. They consider the policy misguided and believe it will not have a positive effect on the caribou population. They also object to the killing of wolves on ethical grounds.

Habitat destruction

Some communities are fragile and easily destroyed by human activity, whereas others seem able to resist human interference. Communities with a wide variety of organisms that show a high level of interaction are more resistant than those with few organisms and little interaction. In general, the more complex an ecosystem is , the more likely it is to recover after being disturbed. The tundra biome is an example of a community with relatively few organisms and interactions. It is not very resistant to change, and because of its slow rate of repair, damage caused by human activity many persist for hundreds of years.

Some species are more resistant to human activity than others. Rabbits, starlings, skunk, and many kinds of insects and plants are able to maintain high populations despite human activity. Indeed, some may even be encouraged by human activity. By contrast, whales, condors, eagles, and many plant and insect species are not able to resist human interference very well. For most of these endangered species it is not humans acting directly with the organisms that cause their endangerment. Very few organisms have been driven to extinction by hunting or direct exploitation. Usually the cause of extinction or endangerment is an indirect effect of habitat destruction as humans exploit natural ecosystems. As humans convert land to farming, grazing, commercial forestry, development, and special wildlife management areas, the natural ecosystems are disturbed, and plants and animals with narrow niches tend to be eliminated because they lose critical resources in their environment. Table 15.1 lists several endangered species and the probable causes of their difficulties.

Pesticide use

Humans have developed a variety of chemicals to control specific pest organisms. One of the first that was used widely was the insecticide DDT. DDT is an abbreviation for the chemical name dichlorodiphenyltrichloroethane. DDT is one of a group of organic compounds called cholorinated hydrocarbons. Because DDT is a poison that was used to kill a variety of insects, it was called an insecticide. Another term that is sometimes used is pesticide, which implies that the poison is effective against pests. Although it is no longer used in the United States (its use was banned in the early 1970s), DDT is still manufactured and used in many parts of the world, include Mexico

     DDT was a valuable insecticide for the U.S. Armed Forces during World War II. It was sprayed on clothing and dusted on the bodies of soldiers, refugees, and prisoners to kill body lice and other insects. Lice, besides being a nuisance, carry the bacteria that can cause a disease known as typhus fever. When bitten by a louse, a person can develop typhus fever. Because body lice could be transferred from one person to another by contact or by wearing infested clothing, DDT was important in maintaining the health of millions of people. Because DDT was so useful in controlling these insects, people envisioned the end of pesky mosquitoes and flies, as well as the elimination of many disease-carrying insects.

Although DDT was originally very effective, many species of insects developed a resistance to it. the genetic diversity present in all species is related to their ability to respond to many environmental factors, including manufactured ones such as DDT. When DDT or any pesticide is applied to a population of insects, susceptible individuals die, and those with some degree of resistance have a greater chance of living. Now the reproducing population consists of many individuals that have resistant genes, which are passed on to the offspring. When this happens repeatedly over a long time, a resistant population develops, and the insecticide is no longer useful.

DDT and other pesticides act as selecting agents, killing the normal insects but allowing the resistant individuals to live. This happened in the orange gloves of California, where many populations of pests became DDT-resistant. Similarly, throughout the world, DDT was used (and in many areas is still used) to control malaria-carrying mosquitoes. Many of these populations have become resistant to DDT and other kinds of insecticides. The people who anticipated the elimination of insect pests did not reckon with the genetic diversity of the gene pools of these insects.

Another problem associated with pesticide use is the effects of pesticides on valuable nontarget organisms. Because many of the insects we consider pests are herbivores, you can expect that carnivores in the community use the pest species as prey, and parasites use the pest as host. These predators and parasites have important roles in controlling the numbers of a pest species.

Genetically, predators and parasites reproduce more slowly then their prey or host species. Because of this, the use of a nonspecific pesticide may indirectly make controlling a pest more difficult. If such a pesticide is applied to an area, the pest is killed but so are it predators and parasites. Because the herbivore pest reproduces faster than its predators and parasites, the pest population rebounds quickly, unchecked by natural predation and parasitism. This may necessitate more frequent and more concentrated applications of pesticides. This has actually happened in many cases of pesticide use; the pesticides made the problem worse, and the chemicals became increasingly costly to apply. Today, a more enlightened approach to pest control involves integrated pest management, which uses a variety of approaches to reduce pest populations. Integrated past management may involve the use of pesticides as part of a pest control program, but it will also include strategies such as encouraging the natural enemies of pests, changing farming practices to discourage pests, changing the mix of crops grown, and accepting low levels of crop damage as an alternative to costly pesticide applications.

biomaginification

another problem associated with the use of persistent chemicals involves their effect on the food chain. DDT was a very effective insecticide because it is extremely toxic to insects but not very toxic to birds and mammals. It is also a very stable compound, which means that once it is applied it remains effective for a long time. It sounds like an ideal insecticide. What went wrong? Why was its use banned?

When DDT was sprayed over an area, it fell on the insects and on the plants that the insects used for food. Eventually the DDT entered the insect either directly through the body wall or through its food. When ingested with food, DDT interferes with the normal bolism of the insect. If small quantities are taken in, the insect can digest and break down the DDT just like any other large organic molecule, because DDT is soluble in fat or oil, the DDT or its break down products are stored in the fat deposits of the insect.

Some insects can break down and store all the DDT they encounter and, therefore, they survive. If an area has been lightly sprayed with DDT, some insects die, some are able to tolerate the DDT, and others break down and store nonlethal quantities of DDT. As much as one part DDT per 1 million parts of insect tissue can be stored in this manner. This is not much DDT! It is equivalent to one drop of DDT in 100 railroad tank cars. However, when an aquatic area is sprayed with small concentration of DDT, many kinds of organisms in the area can accumulate tiny quantities in their bodies. Even algae and protozoa found in aquatic ecosystems accumulate persistent pesticides. They may accumulate concentrations in their cells that are 250 times more concentration than the amount sprayed on the ecosystem. The algae and protozoa are eaten by insects, which in turn are eaten by frogs, fish, or other carnivores.

The concentration in frogs and fish may be 2,000 times what was sprayed. The birds that feed on the frogs and fish may accumulate concentrations that are as much as 80,000 times the original amount. Because DDT is relatively stable and is stored in the fat deposits of the organisms that take it in, what was originally a dilute concentration becomes more concentrated as it moves up the food chain.

Before DDT was banned, many animals at higher trophic levels died as a result of lethal concentrations of pesticide accumulated from the food they ate. Each step in the food chain accumulated some DDT and, therefore, higher trophic levels had higher concentrations. This process is called biomagnification. Even if they were not killed directly by DDT, many birds at higher trophic levels, such as eagles, pelicans, and osprey, suffered reduced populations because the DDT interfered with the female birds’ ability to produce eggshells. Thin eggshells are easily broken, and thus no live young hatched. Both the bald eagle and the brown pelican were placed on the endangered species list because their populations dropped dramatically as a result of DDT poisoning. The ban on DDT use in the United States and Canada has resulted in an increase in the populations of both kinds of birds; the status of the bald eagle has been upgraded from endangered to threatened.

Another widely used group of synthetic compounds of environmental concern are polychloriniated biphenyls (PCBs). PCBs are highly stable compounds that resist changes from heats, acids, bases, and oxidation. These characteristics make PCBs desirable for industrial use,  but also make tem persistent pollutants when released into the environment. About half the PCBs were used in transformers and electrical capacitors. Other uses included inks, plastics, tapes, paints, glues, waxes, and polishes. PCBs are harmful to fish and other aquatic forms of life because they interfere with reproduction. In humans, PCBs produce liver ailments and skin lesions. In high concentrations, they can damage the nervous system and are suspected carcinogens. In 1970, PCB production was limited to cases where satisfactory substitutes were not available. Today, substitutes has been found for nearly all the former uses of PCBs.