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Contents Chemical Biology Part I The Origin of Living Things 1 The Science of Biology Science is the process of testing ideas against observation. Darwin developed his ideas about evolution by testing them against a wealth of observation. In this text science will provide the framework for your exploration of life. 1.1 Biology is the science of life. 1.2 Scientists form generalizations from observations. 1.3 Darwin's theory of evolution illustrates how science works. 1.4 This book is organized to help you learn biology. 2 The Nature of Molecules Organisms are chemical machines, and to understand them we must first learn a little chemistry. We first explore how atoms are linked together into molecules. The character of the water molecule in large measure determines what organisms are like. 2.1 Atoms are nature's building material. 2.2 The atoms of living things are among the smallest. 2.3 Chemical bonds hold molecules together. 2.4 Water is the cradle of life. 3 The Chemical Building Blocks of Life The four kinds of large macromolecules that are the building blocks of organisms are each built up of long chains of carbon atoms. In each, the macromolecule is assembled as a long chain of subunits, like pearls in a necklace or cars of a railway train. 3.1 Molecules are the building blocks of life. 3.2 Proteins perform the chemistry of the cell. 3.3 Nucleic acids store and transfer genetic information. 3.4 Lipids make membranes and store energy. 3.5 Carbohydrates store energy and provide building materials. 4 The Origin and Early History of Life Little is known about how life originated on earth. If it originated spontaneously, as most biologists surmise, then it must have evolved very quickly, as microfossils of bacteria are found in rocks formed soon after earth's surface cooled. 4.1 All living things share key characteristics. 4.2 There are many ideas about the origin of life. 4.3 The first cells had little internal structure. 4.4 The first eukaryotic cells were larger and more complex than bacteria. Cell Biology Part II Biology of the Cell 5 Cell Structure Bacterial cells have little internal organization, while the cells of eukaryotes are subdivided by internal membranes into numerous compartments with different functions. Compartmentalization is the hallmark of the eukaryotic cell. 5.1 All organisms are composed of cells. 5.2 Eukaryotic cells are far more complex than bacterial cells. 5.3 Take a tour of a eukaryotic cell. 5.4 Symbiosis played a key role in the origin of some eukaryotic organelles. 6 Membranes Every cell is encased within a thin membrane that separates it from its environment. The membrane is a mosaic of proteins floating on a sheet of lipid that provide channels into the cell for both molecules and information. 6.1 Biological membranes are fluid layers of lipid. 6.2 Proteins embedded within the plasma membrane determine its character. 6.3 Passive transport across membranes moves down the concentration gradient. 6.4 Bulk transport utilizes endocytosis. 6.5 Active transport across membranes is powered by energy from ATP. 7 Cell-Cell Interactions Cells receive molecular signals with protein receptors on or within the plasma membrane. The information passes into the cell interior as a cascade of interactions that greatly amplify the strength of the original signal. 7.1 Cells signal one another with chemicals. 7.2 Proteins in the cell and on its surface receive signals from other cells. 7.3 Follow the journey of information into the cell. 7.4 Cell surface proteins mediate cell-cell interactions. Cell Biology Part III Energetics 8 Energy and Metabolism Organisms use proteins called enzymes to facilitate chemical reactions. When the products of a reaction contain more energy than the starting materials, the extra amount is supplied by ATP, the energy currency of the cell. 8.1 The laws of thermodynamics describe how energy changes. 8.2 Enzymes are biological catalysts. 8.3 ATP is the energy currency of life. 8.4 Metabolism is the chemical life of a cell. 9 How Cells Harvest Energy Cells harvest chemical energy from the C-H chemical bonds of food molecules. Some of this energy is captured by rearranging chemical bonds, but most of it is harvested by oxidation, in reactions where the electrons of C-H bonds are used to reduce atmospheric oxygen to water. 9.1 Cells harvest the energy in chemical bonds. 9.2 Cellular respiration oxidizes food molecules. 9.3 Catabolism of proteins and fats can yield considerable energy. 9.4 Cells can metabolize food without oxygen. 10 Photosynthesis Photosynthesis is the reverse of respiration, the energy of sunlight being harnessed to reduce carbon dioxide with electrons obtained from water, leaving oxygen gas as the by-product. All organic molecules are the direct or indirect products of photosynthetic carbon fixation. 10.1 What is photosynthesis? 10.2 Learning about photosynthesis: An experimental journey. 10.3 Pigments capture energy from sunlight. 10.4 Cells use the energy and reducing power captured by the light reactions to make organic molecules. Genetics Part IV Reproduction and Heredity 11 How Cells Divide The division of a eukaryotic cell involves a complex and carefully orchestrated division of chromosome copies to the daughter cells. The genes which control cell division are among the most crucial in the genome. Damage to them often results in cancer. 11.1 Bacteria divide far more simply than do eukaryotes. 11.2 Chromosomes are highly ordered structures. 11.3 Mitosis is a key phase of the cell cycle. 11.4 The cell cycle is carefully controlled. 12 Sexual Reproduction and Meiosis Sexual reproduction is only possible because of a special form of cell division called meiosis that reduces the diploid number of chromosomes in half; fertilization then restores the diploid number. Sexual reproduction may have evolved as a way to repair damaged DNA, although other explanations are also being actively considered. 12.1 Meiosis produces haploid cells from diploid cells. 12.2 Meiosis has three unique features. 12.3 The sequence of events during meiosis involves two nuclear divisions. 12.4 The evolutionary origin of sex is a puzzle. 13 Patterns of Inheritance Mendel's theory of heredity rests squarely on the assumption that what is inherited is information rather than the traits themselves. Once researchers understood that the information resided on chromosomes, the reason for Mendelian segregation became clear. 13.1 Mendel solved the mystery of heredity. 13.2 Human genetics follows Mendelian principles. 13.3 Genes are on chromosomes. Genetics Part V Molecular Genetics 14 DNA: The Genetic Material The experiments demonstrating that DNA is the hereditary material are among the most elegant in biology. Its double helical structure leads directly to a mechanism for replicating the molecule that is simple and relatively free of errors. 14.1 What is the genetic material? 14.2 What is the structure of DNA? 14.3 How does DNA replicate? 14.4 What is a gene? 15 Genes and How They Work Gene expression is the mechanism translating the genetic information into the practical reality of what organisms are like. It involves first transcribing a working copy of a gene, then using that copy to direct the assembly of a specific protein. 15.1 The Central Dogma traces the flow of gene-encoded information. 15.2 Genes encode information in three-nucleotide code words. 15.3 Genes are first transcribed, then translated. 15.4 Eukaryotic gene transcripts are spliced. 16 Control of Gene Expression The key to controlling development is to control when particular genes are transcribed. This is done by proteins that can read the DNA double helix without unwinding it, slipping protein segments called "motifs" into the major groove of the double helix. 16.1 Gene expression is controlled by regulating transcription. 16.2 Regulatory proteins read DNA without unwinding it. 16.3 Bacteria limit transcription by blocking RNA polymerase. 16.4 Transcriptional control in eukaryotes operates at a distance. 17 Cellular Mechanisms of Development Vertebrate development is quite different from that of solid worms, insects, or plants, although it shares with these other approaches a common set of basic mechanisms. Detailed study of a few "model systems" has told us a great deal about how development occurs. 17.1 Development is a regulated process. 17.2 Multicellular organisms employ the same basic mechanisms of development. 17.3 Four model developmental systems have been extensively researched. 17.4 Aging can be considered a developmental process. 18 Altering the Genetic Message The genetic message can be altered by mutation, which changes or destroys its content, and by recombination, which alters gene location. Cancer results from mutation of growth-regulating genes, often as the result of cigarette smoking or diet. 18.1 Mutations are changes in the genetic message. 18.2 Cancer results from mutation of growth-regulating genes. 18.3 Recombination alters gene location. 18.4 Genomes are continually evolving. 19 Gene Technology Enzymes that cleave DNA at particular sites have allowed scientists to manipulate the genetic message, creating new combinations of genes that are revolutionizing medicine and agriculture. Every one of us will be affected by gene technology. 19.1 The ability to manipulate DNA has led to a new genetics. 19.2 Genetic engineering involves easily understood procedures. 19.3 Biotechnology is producing a scientific revolution. Evolution and Ecology Part VI Evolution 20 Genes within Populations Evolution occurs within populations when one allele becomes more frequent than another. The study of why allele frequencies change within populations has occupied geneticists for over a century, and a clear picture is now beginning to emerge. 20.1 Genes vary in natural populations. 20.2 Why do allele frequencies change in populations? 20.3 Selection can act on traits affected by many genes. 21 The Evidence for Evolution Adaptive allele changes within populations have been demonstrated many times, and a variety of evidence, much of it compelling, argues that these changes lead to the macroevolutionary changes familiar to us as the fossil record. 21.1 Fossil evidence indicates that evolution has occurred. 21.2 Natural selection can produce evolutionary change. 21.3 Evidence for evolution can be found in other fields of biology. 21.4 The theory of evolution has proven controversial. 22 The Origin of Species Species, the basic units of evolution, originate when two populations of a species adapt to different environments. Selection will tend to favor changes that inhibit gene flow between them, eventually creating a genetic barrier that isolates the populations. 22.1 Species are the basic units of evolution. 22.2 Species maintain their genetic distinctiveness through barriers to reproduction. 22.3 We have learned a great deal about how species form. 22.4 Clusters of species reflect rapid evolution. 23 How Humans Evolved Humans evolved in Africa from a bipedal kind of ape called an australopithecine some two million years ago. The first humans to leave Africa, H. erectus, spread across the earth over a million years ago. How modern humans replaced them is a matter of some controversy. 23.1 The evolutionary path to humans starts with the advent of primates. 23.2 The first hominids to evolve were australopithecines. 23.3 The genus Homo evolved in Africa. 23.4 Modern humans evolved quite recently. Part VII Ecology and Behavior 24 Population Ecology The way in which a population grows depends importantly upon how many young individuals it contains. The way in which a species reproduces represents an evolutionary trade-off between reproductive cost and investment in survival. 24.1 Populations are individuals of the same species that live together. 24.2 Population dynamics depend critically upon age distribution. 24.3 Life histories often reflect trade-offs between reproduction and survival. 24.4 Population growth is limited by the environment. 24.5 The human population has grown explosively in the last three centuries. 25 Community Ecology Organisms make complex evolutionary adjustments to living together within communities. Some adjustments involve cooperation, others capturing prey or avoiding being captured. 25.1 Interactions among competing species shape ecological niches. 25.2 Predators and their prey coevolve. 25.3 Evolution sometimes fosters cooperation. 25.4 Ecological succession may increase species richness. 26 Animal Behavior The study of animal behavior has historically been carried out in two quite different ways, which are only now merging into a unified view. One stressed fixed behaviors constrained by neural organization, the other flexible behaviors influenced by learning. 26.1 Ethology focuses on the natural history of behavior. 26.2 Comparative psychology focuses on how learning influences behavior. 26.3 Communication is a key element of many animal behaviors. 26.4 Migratory behavior presents many puzzles. 26.5 To what degree animals "think" is a subject of lively dispute. 27 Behavioral Ecology Much of the excitement in behavioral science today comes from analysis of how evolution has shaped, and is shaping, the behavior of animal species in natural populations. Often quite controversial, these studies combine theory and careful field observation. 27.1 Evolutionary forces shape behavior. 27.2 Reproductive behavior involves many choices influenced by natural selection. 27.3 There is considerable controversy about the evolution of social behavior. 27.4 Vertebrates exhibit a broad range of social behaviors. Evolution and Ecology Part VIII The Global Environment 28 Dynamics of Ecosystems Ecological systems are dynamic "machines" in which chemicals cycle between organisms and the environment and energy is used as it passes through the system. Much energy is lost to heat at each step of its journey through living things. In general, communities with more species interacting with one another are more stable. 28.1 Chemicals cycle within ecosystems. 28.2 Ecosystems are structured by who eats whom. 28.3 Energy flows through ecosystems. 28.4 Biodiversity promotes ecosystem stability. 29 The Biosphere The sun powers major movements of the atmosphere that circulate heat and moisture over the surface of the globe, creating different climates in different locales. Each characteristic climate favors a particular community of organisms adapted to living there. 29.1 Organisms must cope with a varied environment. 29.2 Climate shapes the character of ecosystems. 29.3 Biomes are widespread terrestrial ecosystems. 29.4 Aquatic ecosystems cover much of the earth. 30 The Future of the Biosphere The world's human population is growing at an explosive rate, placing a severe strain on the world's ecosystems. No one knows if the world can sustain the 6 billion people it now contains-and the population will soon grow much larger. 30.1 The world's human population is growing explosively. 30.2 Improvements in agriculture are needed to feed a hungry world. 30.3 Human activity is placing the environment under increasing stress. 30.4 Solving environmental problems requires individual involvement. 31 Conservation Biology The successive efforts to halt the world's alarming loss of biodiversity will depend critically on our gaining a better understanding of what forces threaten species survival, and how they can be counteracted. 31.1 The new science of conservation biology is focused on conserving biodiversity. 31.2 Vulnerable species are more likely to become extinct. 31.3 Causes of endangerment usually reflect human activities. 31.4 Successful recovery plans will need to be multidimensional. Simple Organisms Part IX Viruses and Simple Organisms 32 How We Classify Organisms The living world is a rich tapestry of diversity, teeming with different kinds of organisms. One of the great challenges of biology is to find sensible ways to name and classify kinds of organisms, ways that tell us about them and how they relate to each other. 32.1 Biologists name organisms in a systematic way. 32.2 Scientists construct phylogenies to understand the evolutionary relationships among organisms. 32.3 All living organisms are grouped into one of a few major categories. 33 Viruses Viruses are not considered organisms because they lack cellular structure. However, because they can replicate themselves within the cells of organisms, viruses are able to evolve, and are responsible for a broad array of serious diseases. 33.1 Viruses are strands of nucleic acid encased within a protein coat. 33.2 Bacterial viruses exhibit two sorts of reproductive cycles. 33.3 HIV is a complex animal virus. 33.4 Non-living infectious agents are responsible for many human diseases. 34 Bacteria Bacteria are the simplest organisms, composed of single cells that lack the complex internal organization seen in all other cells. Biologists believe bacteria were the first organisms to evolve on earth, and they are easily the most numerous and successful. 34.1 Bacteria are the smallest and most numerous organisms. 34.2 Bacterial cell structure is more complex than commonly supposed. 34.3 Bacteria exhibit considerable diversity in both structure and metabolism. 34.4 Bacteria are responsible for many diseases but also make important contributions to ecosystems. 35 Protists All eukaryotic organisms that are not fungi, plants, or animals are lumped together in a catch-all category called protists. Except for the marine algae, all protists are unicellular, but the diversity among protists is so great that it is difficult to compare them. 35.1 Eukaryotes probably arose by endosymbiosis. 35.1 The kingdom Protista is by far the most diverse of any kingdom. 35.2 Protists can be categorized into five groups. 36 Fungi Fungi are extraordinarily strange multicellular organisms whose cells share cytoplasm and nuclei. Fungi absorb their food from their surroundings, first excreting digestive enzymes, then soaking up the resulting soup of molecular fragments. 36.1 Fungi are unlike any other kind of organism. 36.2 Fungi are classified by their reproductive structures. 36.3 Fungi form two key mutualistic symbiotic associations. Plants Part X Plant Form and Function 37 Evolutionary History of Plants Plants are responsible for much of the photosynthesis that supports life on earth. Early plants resembled nonvascular mosses in having no internal structures to move water up and down the stem. Their vascular descendants added seeds and then flowers. 37.1 Plants have multicellular haploid and diploid stages in their life cycles. 37.2 Nonvascular plants are relatively unspecialized but successful in many terrestrial environments. 37.3 Seedless vascular plants have well-developed conducting tissues in their sporophytes. 37.4 Seeds protect and aid in the dispersal of plant embryos. 38 The Plant Body A plant is basically a tubular stem with roots attached to the bottom and leaves to the top. Growth occurs at the tip of the shoot, at the ends of the roots, and in a sheath around the plant stem. Roots gather in nutrients from the soil, while leaves carry out photosynthesis. 38.1 Meristems elaborate the plant body plan after germination. 38.2 Plants have three basic tissues, each composed of several cell types. 38.3 Root cells differentiate as they become distanced from the dividing root apical meristem. 38.4 Stems are the backbone of the shoot, transporting nutrients and supporting the aerial plant organs. 38.5 Leaves are adapted to support basic plant functions. 39 Nutrition and Transport in Plants Plants require a variety of mineral nutrients that they obtain from the soil. Nutrients are carried from the roots to the leaves dissolved in water, which is sucked up the stem through the xylem by transpiration from the leaves. Carbohydrates are carried from the leaves to the roots dissolved in water that travels down the phloem. 39.1 Plants require a variety of nutrients in addition to the direct products of photosynthesis. 39.2 Some plants have novel strategies for obtaining nutrients. 39.3 Water and minerals move upward through the xylem. 39.4 Dissolved sugars and hormones are transported in the phloem. Part XI Plant Growth and Reproduction 40 Early Plant Development The plant body develops in modules of leaf, shoot, and root. The course of the plant body's development is strongly influenced by the environment. Many plant structures, including seeds and fruits, have evolved to aid dispersal of offspring to new locations. 40.1 Plant embryo development establishes a basic body plan. 40.2 The seed protects the dormant embryo from water loss. 40.3 Fruit formation enhances the dispersal of seeds. 40.4 Germination initiates post-seed development. 41 How Plants Grow in Response to Their Environment Every plant cell contains a full set of hereditary information. Which genes are expressed is controlled by a set of plant hormones, including auxin, ethylene, and many others. These hormones interact with each other and with the environment to determine the pattern of growth. 41.1 Plant growth is often guided by environmental cues. 41.2 The hormones that guide growth are keyed to the environment. 41.3 The environment influences flowering. 41.4 Many short-term responses to the environment do not require growth. 42 Plant Reproduction Many plants can clone themselves by asexual reproduction. Some plants use the wind to transport pollen from male to female. A much more efficient delivery system is employed by flowering plants, which use insects to carry pollen from flower to flower. 42.1 Angiosperms have been incredibly successful, in part, because of their reproductive strategies. 42.2 Flowering plants use animals or wind to transfer pollen between flowers. 42.3 Many plants can clone themselves by asexual reproduction. 42.4 How long do plants and plant organs live? 43 Plant Genomics Plants organize their hereditary material in a more complex way than animals, with extensive regions of repetitive DNA and often many copies of chromosomes. Cloning plants in tissue culture and altering their genes have led to major agricultural advances. 43.1 Genomic organization is much more varied in plants than in animals. 43.2 Advances in plant tissue culture are revolutionizing agriculture. 43.3 Plant biotechnology now affects every aspect of agriculture. Animals Part XII Animal Diversity 44 The Noncoelomate Animals The simplest animals lack a coelomic body cavity. The most ancient are the sponges, which lack tissues. Many of these evolutionarily old noncoelomate animals are called "worms," but this simple designation hides a great diversity of form and function. 44.1 Animals are multicellular heterotrophs without cell walls. 44.2 The simplest animals are not bilaterally symmetrical. 44.3 Acoelomates are solid worms that lack a body cavity. 44.4 Pseudocoelomates have a simple body cavity. 44.5 The coming revolution in animal taxonomy will likely alter traditional phylogenies. 45 Mollusks and Annelids Mollusks and annelid worms, both of which originated in the sea and have similar larvae, are also very successful on land. Indeed, there are more terrestrial species of mollusks than there are terrestrial vertebrate species! 45.1 Mollusks were among the first coelomates. 45.2 Annelids were the first segmented animals. 45.3 Lophophorates appear to be a transitional group. 46 Arthropods Arthropods are easily the most successful animal group, particularly the insects. Along with the jointed appendages that are the hallmark of arthropods, insects evolved wings. There are more species of beetles than there are of any nonarthropod animal phylum. 46.1 The evolution of jointed appendages has made arthropods very successful. 46.2 The chelicerates all have fangs or pincers. 46.3 Crustaceans have branched appendages. 46.4 Insects are the most diverse of all animal groups. 47 Echinoderms The vertebrates closest relatives among the animals are echinoderms, what most people think of as "starfish." Like vertebrates, they have deuterostome development and an endoskeleton. Unlike vertebrates, however, echinoderms are radially symmetrical as adults. 47.1 The embryos of deuterostomes develop quite differently from those of protostomes. 47.2 Echinoderms are deuterostomes with an endoskeleton. 47.3 The six classes of echinoderms are all radially symmetrical as adults. 48 Vertebrates Vertebrates are members of the phylum Chordata, all of whose members have a notochord at some stage in their development. The hallmark of vertebrates is an interior skeleton of bone which provides a superb framework for muscle attachment. 48.1 Attaching muscles to an internal framework greatly improves movement. 48.2 Nonvertebrate chordates have a notochord but no backbone. 48.3 The vertebrates have an interior framework of bone. 48.4 The evolution of vertebrates involves invasions of sea, land, and air. Part XIII Animal Form and Function 49 Organization of the Animal Body Although they at first glance seem quite different from one another, all vertebrates are basically similar in body design, using the same array of tissues to form a similar array of organs. Vertebrates differ principally in the degree of terrestrial adaptation. 49.1 The bodies of vertebrates are organized into functional systems. 49.2 Epithelial tissue forms membranes and glands. 49.3 Connective tissues contain abundant extracellular material. 49.4 Muscle tissue provides for movement, and nerve tissue provides for control. 50 Locomotion Muscle tissue, because it can contract, allows animals to move about. Some animals, like fish and snakes, move their entire bodies, while others move limbs such as arms, legs, or wings. The contraction of muscle is powered by ATP and controlled by the nervous system. 50.1 A skeletal system supports movement in animals. 50.2 Skeletal muscles contract to produce movements at joints. 50.3 Muscle contraction powers animal locomotion. 51 Fueling Body Activities: Digestion Before the cells of an animal can assimilate fatty acids, sugars, and amino acids to use in manufacturing ATP, these "food" molecules must be cleaved out of far larger fats, carbohydrates, and proteins. This process of digestion takes place in the stomach and small intestine. 51.1 Animals employ a digestive system to prepare food for assimilation by cells. 51.2 Food is ingested, swallowed, and transported to the stomach. 51.3 The small and large intestines have very different functions. 51.4 Accessory organs, neural stimulation, and endocrine secretions assist in digestion. 51.5 All animals require food energy and essential nutrients. 52 Circulation The highway that carries materials from one place to another in the vertebrate body is a network of flexible pipes called arteries and veins. A muscular pump called the heart pushes a rich fluid called blood through these pipes to every organ in the body. 52.1 The circulatory systems of animals may be open or closed. 52.2 A network of vessels transports blood through the body. 52.3 The vertebrate heart has undergone progressive evolutionary change. 52.4 The cardiac cycle drives the cardiovascular system. 53 Respiration One of the most important cargos carried by blood are two gases, oxygen and carbon dioxide. In the lungs, blood picks up oxygen from the air and dumps carbon dioxide acquired from the body's tissues as a by-product of their respiratory metabolism. 53.1 Respiration involves the diffusion of gases. 53.2 Gills are used for respiration by aquatic vertebrates. 53.3 Lungs are used for respiration by terrestrial vertebrates. 53.4 Mammalian breathing is a dynamic process. 53.5 Blood transports oxygen and carbon dioxide. Animals Part XIV Regulating the Animal Body 54 The Nervous System The activities of the many organs of the vertebrate body are coordinated by the nervous system, an extensive network of neuron cells connecting all organs of the body. At a central coordinating center, the brain, information is processed, associations made, and commands given. 54.1 The nervous system consists of neurons and supporting cells. 54.2 Nerve impulses are produced on the axon membrane. 54.3 Neurons form junctions called synapses with other cells. 54.4 The central nervous system consists of the brain and spinal cord. 54.5 The peripheral nervous system consists of sensory and motor neurons. 55 Sensory Systems Information used by the nervous system to coordinate the body's activities comes from a system of sensors that collects information about the body's condition, its position in space, and what is going on around it. Vertebrates differ substantially in which sensors they employ. 55.1 Animals employ a wide variety of sensory receptors. 55.2 Mechanical and chemical receptors sense the body's condition. 55.3 Auditory receptors detect pressure waves in the air. 55.4 Optic receptors detect light over a broad range of wavelengths. 55.5 Some vertebrates use heat, electricity, or magnetism for orientation. 56 The Endocrine System The vertebrate nervous system uses long-lasting chemical signals called hormones to coordinate many physiological and developmental processes. The cells that secrete these hormones are often regulated by a set of command hormones released by the brain. 56.1 Regulation is often accomplished by chemical messengers. 56.2 Lipophilic and polar hormones regulate their target cells by different means. 56.3 The hypothalamus controls the secretions of the pituitary gland. 56.4 Endocrine glands secrete hormones that regulate many body functions. 57 The Immune System Vertebrates defend themselves from infection by viruses, bacteria, and other microbes with an army of circulating cells that seek out and check the identity of all cells in the body. When a stranger is identified, other "killer" cells are called in to deal with the invader. 57.1 Many of the body's most effective defenses are nonspecific. 57.2 Specific immune defenses require the recognition of antigens. 57.3 T cells organize attacks against invading microbes. 57.4 B cells label specific cells for destruction. 57.5 All animals exhibit nonspecific immune response but specific ones evolved invertebrates. 57.6 The immune system can be defeated. 58 Maintaining the Internal Environment Vertebrates maintain internal conditions at constant values, often quite different from those of their environment. This is particularly true of temperature and salt levels. Vertebrates go to great pains to maintain body temperature and avoid water loss or gain. 58.1 The regulatory systems of the body maintain homeostasis. 58.2 The extracellular fluid concentration is constant in most vertebrates. 58.3 The functions of the vertebrate kidney are performed by nephrons. 58.4 The kidney is regulated by hormones. 59 Sex and Reproduction 1 Vertebrates usually reproduce sexually, although asexual reproduction is not impossible for them. In the sea, females release eggs into the water for external fertilization by the males' sperm. On land, evolution has favored internal fertilization. 59.1 Animals employ both sexual and asexual reproductive strategies. 59.2 The evolution of reproduction among the vertebrates has led to internalization of fertilization and development. 59.3 Male and female reproductive systems are specialized for different functions. 59.4 The physiology of human sexual intercourse is becoming better known. 60 Vertebrate Development In vertebrates, the path of development from fertilization to birth is well known. The development of the embryo undergoes its key stages early on, determining the basic architecture of the body and its tissues. Thereafter, much of what occurs is growth. 60.1 Fertilization is the initial event in development. 60.2 Cell cleavage and the formation of a blastula set the stage for later development. 60.3 Gastrulation forms the three germ layers of the embryo. 60.4 Body architecture is determined during the next stages of embryonic development. 60.5 Human development is divided into trimesters.
Library of Congress Subject Headings for this publication: Biology