Zoology

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Zoology

It is the branch of biology devoted to the study of the animal kingdom (Animalia). This article discusses the history and concerns of that study. For a discussion of animals and a description of animal groups.

History of  zoology

The study of zoology can be viewed as a series of efforts to analyze and classify animals. Attempts at classification as early as 400 bc are known from documents in the Hippocratic Collection. Aristotle, however, was the first to devise a system of classifying animals that recognized a basic unity of plan among diverse organisms; he arranged groups of animals according to mode of reproduction and habitat. Observing the development of such animals as the dogfish, chick, and octopus, he noted that general structures appear before specialized ones, and he also distinguished between asexual and sexual reproduction. His Historia Animalium contains accurate descriptions of extant animals of Greece and Asia Minor. He was also interested in form and structure and concluded that different animals can have similar embryological origins and that different structures can have similar functions.

In Roman times Pliny the Elder compiled four volumes on zoology in his 37-volume treatise called Historia Naturalis. Although widely read during the Middle Ages, they are little more than a collection of folklore, myth, and superstition. One of the more influential figures in the history of physiology, the Greek physician Galen, dissected farm animals, monkeys, and other mammals and described many features accurately, although some were wrongly applied to the human body. His misconceptions, especially with regard to the movement of blood, remained virtually unchanged for hundreds of years. In the 17th century, the English physician William Harvey established the true mechanism of blood circulation.

Until the Middle Ages, zoology was a conglomeration of folklore, superstition, misconception, and descriptions of animals, but during the 12th century it began to emerge as a science. Perhaps the most important naturalist of the era was the German scholar St. Albertus Magnus, who denied many of the superstitions associated with biology and reintroduced the work of Aristotle. The anatomical studies of Leonardo da Vinci were far in advance of the age. His dissections and comparisons of the structure of humans and other animals led him to important conclusions. He noted, for example, that the arrangement of joints and bones in the leg are similar in both horses and humans, thus grasping the concept of homology (the similarity of corresponding parts in different kinds of animals, suggesting a common grouping). The value of his work in anatomy was not recognized in his time. Instead, the Belgian physician Andreas Vesalius is considered the father of anatomy; he circulated his writings and established the principles of comparative anatomy.

Classification dominated zoology throughout most of the 17th and 18th centuries. The Swedish botanist Carolus Linnaeus developed a system of nomenclature that is still used today—the binomial system of genus and species (see Classification) —and established taxonomy as a discipline. He followed the work of the English naturalist John Ray in relying upon the form of teeth and toes to differentiate mammals and upon beak shape to classify birds. Another leading systematist of this era was the French biologist Comte Georges Leclerc de Buffon. The study of comparative anatomy was extended by such men as Georges Cuvier, who devised a systematic organization of animals based on specimens sent to him from all over the world.

Current study of zoology

In the 20th century zoology has become more diversified and less confined to such traditional concerns as classification and anatomy. Broadening its range to include such studies as genetics, ecology, and biochemistry, zoology has become an interdisciplinary field applying a great variety of techniques to obtain knowledge of the animal kingdom.

Taxonomically oriented studies concentrate on the different divisions of animal life. Invertebrate zoology deals with multicellular animals without backbones; its subdivisions include Entomology (the study of insects) and Malacology (the study of mollusks). Vertebrate zoology, the study of animals with backbones, is divided into ichthyology (fish), herpetology (amphibians and reptiles), ornithology (birds), and mammalogy (mammals).

Malacology

Entomology

Paleontology, the study of fossils, is subdivided by taxonomic groups. In each of these fields, researchers investigate the classification, distribution, life cycle, and evolutionary history of the particular animal or group of animals under study. Most zoologists are also specialists in one or more of the process-oriented disciplines.

Morphology, the study of structure, includes gross morphology, which examines entire structures or systems, such as muscles or bones; histology, which examines body tissues; and cytology, which focuses on cells and their components. Many great advances made in cytology in recent years are attributable to the electron microscope and the scanning electron microscope. Special staining techniques and radioactive tracers have been used to differentiate structural detail at the molecular level. Methods have been developed for mapping neural connections between parts of the brain and for stimulating and recording impulses from specific brain sites and even individual nerve cells.

Physiology, the study of function, is closely associated with morphology. An important subdivision is cellular physiology, which is closely related to molecular biology. Another active field, physiological ecology, studies the physical responses of animals to their environment. Much of this work has been carried out on desert, arctic, and ocean animals that must survive extremes of temperature or pressure.

Animal behavioral  studies developed along two lines. The first of these, animal psychology, is primarily concerned with physiological psychology and has traditionally concentrated on laboratory techniques such as conditioning. The second,   the most important recent development in the field is the concentration on socio-biology, which is concerned with the behaviour, ecology, and evolution of social animals such as bees, ants, schooling fish, flocking birds, and humans.Sociobiology is still in its infancy and is quite controversial, chiefly because it has raised anew the old dispute about whether behaviour is genetically determined.

Embryology, the study of the development of individual animals, has investigated the way in which developing parts interact—for example, the interactions between the eyestalk and the epidermis during development of the lens of the eye. The emerging field of molecular development applies the techniques of molecular biology, including molecular genetics.

The study of the interactions between animals and their environment is known as Ecology. Primary attention is given to the complex pattern of interactions among the many species constituting a community. Ecology has been central to the development of conservation and environmental control during the past 20 years. It has revealed the deleterious effects of pesticides and industrial pollutants and has provided important insights into wiser management of agriculture, forestry, and fisheries.

Evolutionary Zoology, which draws on all of the fields just mentioned, is concerned with the mechanisms of evolutionary change—speciation and adaptation—and with the evolutionary history of animal groups. Particularly relevant to evolutionary studies are systematics, phylogenetics, paleontology, and zoogeography. Systematics deals with the delineation and description of animal species and with their arrangement into a classification. Phylogenetics is the study of the developmental history of groups of animals. Zoogeography, the study of the distribution of animals over the earth, is closely related to ecology and systematics. See Animal Distribution; Paleontology; Species and Speciation.

BOTANY

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Botany

Botany, branch of biology concerned with the study of plants (kingdom Plantae; see Plant). Plants are now defined as multicellular organisms that carry out photosynthesis. Organisms that had previously been called plants, however, such as bacteria, algae, and fungi, continue to be the province of botany, because of their historical connection with the discipline and their many similarities to true plants, and because of the practicality of not fragmenting the study of organisms into too many separate fields.

HISTORICAL DEVELOPMENT

The Greeks believed that plants derived their nourishment from the soil only. Not until the 17th century did the Belgian scientist Jan Baptista van Helmont show that, although only water was added to a potted willow, it gained nearly 75 kg (165 lb), whereas the soil it stood in lost only about 60 g (about 2 oz) of weight over a period of five years. This demonstrated that the soil contributes very little to the increase in the weight of plants. In the 18th century the English chemist Joseph Priestley demonstrated that growing plants “restore” air from which the oxygen has been removed (by the burning of candles or the breathing of animals), and the Dutch physiologist Jan Ingenhousz (1730-99) extended this observation by showing that light is required for plants to restore air. These and other discoveries formed the basis for modern plant physiology, that branch of botany dealing with basic plant functions.

That water moves upward through the wood and that solutes move downward through the stems of plants was discovered independently in the 17th century by Marcello Malpighi in Italy and Nehemiah Grew in England. These facts have now been known for some 300 years, but only in the last few years have acceptable theories explaining the movements of liquids in plants been developed, using a variety of refined analytical techniques.

CLASSICAL STUDIES

Gross observations and experiments on photosynthesis and the movement of water in plants can be made without knowledge of their structure, but explanations of these phenomena require knowledge of morphology—the study and interpretation of plant form, development, and life histories—and of anatomy—the study of plant tissues and their origin and relations to one another. The cellular nature of plants was first pointed out by the English scientist Robert Hooke in the 17th century, when he observed that cork bark consists of cells. In 1838 the German botanist Matthias Schleiden proposed that all plant tissues consist of cells; this implied a basic sameness of living things and laid the foundation for the development of cytology, the study of the structure and function of cells as individual units rather than as aggegrate tissue. The German pathologist Rudolf Virchow showed in 1858 that cells are derived from preexisting cells, and thus that a continuity exists between past and present living things.

BOTANY TODAY

Botany does not depend on the fossil record for information concerning evolution and classification as much as does zoology, because the record for plants is much less complete than that for animals. Nevertheless, paleobotany, the study of fossil plants, has contributed greatly to the overall understanding of the evolution of the major groups of plants and especially to understanding of the interrelationships among the classes of seed plants. But much remains to be learned before fundamental questions such as the origin of the flowering plants (see Angiosperm) can be answered.

Botanists—those engaged in the study of plants—occupy themselves with a broad range of activities. Many botanists are in academic positions that involve both teaching and research duties. The latter may involve laboratory work or field studies. Strictly speaking, botany is a pure science concerned with investigating the basic nature of plants. Many aspects of botany, however, have direct importance to human welfare and advancement, and applied botany is an important field. Such fields as forestry and horticulture are closely tied to basic botanical studies, whereas those such as pharmacology and agronomy are not as closely related but still depend on basic botanical knowledge.

Scientific Method

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Scientific Method

Whatever the aim of their work, scientists use the same underlying steps to organize their research: (1) they make detailed observations about objects or processes, either as they occur in nature or as they take place during experiments; (2) they collect and analyze the information observed; and (3) they formulate a hypothesis that explains the behavior of the phenomena observed. 

Observation and Experiments

A scientist begins an investigation by observing an object or an activity. Observation typically involves one or more of the human senses—hearing, sight, smell, taste, and touch. Scientists typically use tools to aid in their observations. Scientists typically apply their observation skills to an experiment. An experiment is any kind of trial that enables scientists to control and change at will the conditions under which events occur. It can be something extremely simple, such as heating a solid to see when it melts, or something highly complex, such as bouncing a radio signal off the surface of a distant planet. Scientists typically repeat experiments, sometimes many times, in order to be sure that the results were not affected by unforeseen factors.

Most experiments involve real objects in the physical world, such as electric circuits, chemical compounds, or living organisms. However, with the rapid progress in electronics, computer simulations can now carry out some experiments instead. If they are carefully constructed, these simulations or models can accurately predict how real objects will behave.

Data Collection and Analysis

During an experiment, scientists typically make measurements and collect results as they work. This information, known as data, can take many forms. Data may be a set of numbers, such as daily measurements of the temperature in a particular location or a description of side effects in an animal that has been given an experimental drug. Scientists typically use computers to arrange data in ways that make the information easier to understand and analyze. Data may be arranged into a diagram such as a graph that shows how one quantity (body temperature, for instance) varies in relation to another quantity (days since starting a drug treatment). A scientist flying in a helicopter may collect information about the location of a migrating herd of elephants in Africa during different seasons of a year. The data collected may be in the form of geographic coordinates that can be plotted on a map to provide the position of the elephant herd at any given time during a year.

Scientists use mathematics to analyze the data and help them interpret their results. The types of mathematics used include statistics, which is the analysis of numerical data, and probability, which calculates the likelihood that any particular event will occur.

Formulating the Hypothesis

Once an experiment has been carried out and data collected and analyzed, scientists look for whatever pattern their results produce and try to formulate a hypothesis that explains all the facts observed in an experiment. In developing a hypothesis, scientists employ methods of induction to generalize from the experiment’s results to predict future outcomes, and deduction to infer new facts from experimental results.

Formulating a hypothesis may be difficult for scientists because there may not be enough information provided by a single experiment, or the experiment’s conclusion may not fit old theories. Sometimes scientists do not have any prior idea of a hypothesis before they start their investigations, but often scientists start out with a working hypothesis that will be proved or disproved by the results of the experiment. Scientific hypotheses can be useful, just as hunches and intuition can be useful in everyday life. But they can also be problematic because they tempt scientists, either deliberately or unconsciously, to favor data that support their ideas. Scientists generally take great care to avoid bias, but it remains an ever-present threat. Throughout the history of science, numerous researchers have fallen into this trap, either in the hope of self-advancement or because they firmly believe their ideas to be true. 

CELL STRUCTURE

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Cell Membrane

Most of the lipids in the plasma membrane are of a specific type known as phospholipids. A phospholipid molecule has a head region at one end that is hydrophilic—it can mix with water. At the other end are two long tails that are hydrophobic—they do not mix well with water. In the plasma membrane’s bilayer construction, phospholipid molecules are arranged so that their hydrophilic heads point outward on either side of the membrane, and their hydrophobic tails point toward each other in the middle of the membrane. This orientation keeps the hydrophobic tails away from the watery fluids that both fill and surround living cells. In fact, the plasma membrane stays intact precisely because the phospholipid molecules strongly resist any change in configuration that would expose their hydrophobic tails to the watery environment.

While the phospholipids are held in a bilayer, scientists believe the plasma membrane as a whole is a fluid structure because phospholipid molecules and some proteins can move sideways within the membrane. In one second, a single phospholipid molecule can travel the length of a large bacterial cell. Proteins drift more slowly through the membrane. With protein molecules scattered among the phospholipid molecules, the plasma membrane appears to be a mosaic of phospholipids and proteins. Some of the proteins are found on the inner or outer surface of the plasma membrane, while others span the membrane and protrude on either end. Scientists refer to this concept of the plasma membrane’s structure as the fluid mosaic model.

Function

The plasma membrane forms an extremely effective seal around the cell. Only a very few molecules can pass directly through the lipid bilayer to get from one side of the membrane to the other. Many substances that a cell needs in order to survive cannot cross the lipid bilayer on their own, including glucose (a sugar that cells burn for energy), amino acids (the building blocks of proteins), and ions, such as sodium and potassium. A cell uses two methods to move such substances from one side of the plasma membrane to another, known as passive transport and active transport. Both of these processes involve proteins in the plasma membrane.

Cytoplasm

The most extensive organelle in the cell is the cytoskeleton, a web of protein filaments that branches extensively throughout the cytoplasm and gives the cell its shape. The cytoskeleton proteins, as well as other proteins in the cell, are made by tiny spherical organelles known as ribosomes. Several other important organelles are found in the cells. Among them are the lysosomes, membranous sacs storing enzymes that digest and recycle worn out cell parts; and the mitochondria, sacs where the cell's energy is generated. The endoplasmic reticulum, another organelle, is an extensive network of membrane folds and tubes that serves in part as the cell's factory floor where large molecules, such as lipids, are manufactured. These large molecules are sent to another organelle, the Golgi apparatus, which consists of layers of membranes where the molecules are modified, sorted, and packaged for transport.

Nucleus

The largest and most conspicuous organelle is the nucleus. The nucleus encloses and protects the cell’s genetic material, deoxyribonucleic acid (DNA), so that it is not damaged by biochemical reactions in the cell. Within the eukaryotic nucleus, DNA is wrapped around specialized proteins called histones, like a thread wound around a series of spools. Each DNA strand and its histones fold back and forth several times to form a compact, stick-shaped structure called a chromosome. Depending on the organism, the nucleus contains from one to over a thousand chromosomes. Surrounding the nucleus is the nuclear envelope, a membrane with numerous pores. The pores, ringed by special protein, regulate the flow of substances into and out of the nucleus.

cell Division

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Cell Division

Most cells divide at some time during their life cycle, and some divide dozens of times before they die. Organisms rely on cell division for reproduction, growth, and repair and replacement of damaged or worn out cells. Three types of cell division occur: binary fission, mitosis, and meiosis. Binary fission, the method used by prokaryotes, produces two identical cells from one cell. The more complex process of mitosis, which also produces two genetically identical cells from a single cell, is used by many unicellular eukaryotic organisms for reproduction. Multicellular organisms use mitosis for growth, cell repair, and cell replacement. In the human body, for example, an estimated 25 million mitotic cell divisions occur every second in order to replace cells that have completed their normal life cycles. Cells of the liver, intestine, and skin may be replaced every few days. Recent research indicates that even brain cells, once thought to be incapable of mitosis, undergo cell division in the part of the brain associated with memory.

Mitosis

It is process in which a cell’s nucleus replicates and divides in preparation for division of the cell. Mitosis results in two cells that are genetically identical, a necessary condition for the normal functioning of virtually all cells. Mitosis is vital for growth; for repair and replacement of damaged or worn out cells; and for asexual reproduction, or reproduction without eggs and sperm.

Mitosis occurs in five steps: prophase, prometaphase, metaphase, anaphase, and telophase .

 In prophase the replicated, linked DNA strands slowly wrap around proteins that in turn coil and condense into two short, thick, rodlike structures called chromatids, attached by the centromere. Two structures called centrioles, both located on one side of the nucleus, separate and move toward opposite poles of the cell. As the centrioles move apart, they begin to radiate thin, hollow, proteins called microtubules. The microtubules arrange themselves in the shape of a football, or spindle, that spans the cell, with the widest part at the center of the cell and the narrower ends at opposite poles.

Prometaphase is marked by the disintegration of the nuclear membrane. As the spindle forms, the nuclear membrane breaks down into tiny sacs or vesicles that are dispersed in the cytoplasm. The spindle fibers attach to the chromatids near the centromeres, and tug and push the chromatids so that they line up in the equatorial plane of the cell halfway between the poles. Like two individuals standing back to back at the equator, one chromatid faces one pole of the cell, and its linked partner faces the opposite pole.

In metaphase, exactly half of the chromatids face one pole, and the other half face the other pole. This equilibrium position is called the metaphase plate.

Anaphase begins when the centromeres split, separating the identical chromatids into single chromosomes, which then move along the spindle fibers to opposite poles of the cell. At the end of anaphase, two identical groups of single chromosomes congregate at opposite poles of the cell.

In telophase, the final stage of mitosis, a new nuclear membrane forms around each new group of chromosomes. The spindle fibers break down and the newly formed chromosomes begin to unwind. If viewed under a light microscope, the chromosomes appear to fade away. They exist, however, in the form of chromatin, the extended, thin strands of DNA too fine to be seen except with electron microscopes. Mitosis accomplishes replication and division of the nucleus.

Cytokinesis

The final phase of the cell cycle is known as cytokinesis. The timing of cytokinesis varies depending on the cell type. It can begin in anaphase and finish in telophase; or it can follow telophase. In cytokinesis, the cell’s cytoplasm separates in half, with each half containing one nucleus. Animals and plants accomplish cytokinesis in slightly different ways. In animals, the cell membrane pinches in, creating a cleavage furrow, until the mother cell is pinched in half. In plants, cellulose and other materials that make up the cell wall are transported to the midline of the cell and a new cell wall is constructed. The process of DNA replication, the precise alignment of the chromosomes in mitosis, and the successful separation of identical chromatids in anaphase results in two new cells that are genetically identical. The new cells enter interphase, and the cell cycle begins again.

Meiosis, process of cell division in which the cell’s genetic information, contained in chromosomes, is mixed and divided into sex cells with half the normal number of chromosomes. The sex cells can later combine to form offspring with the full number of chromosomes. The random sorting of chromosomes during meiosis assures that each new sex cell, and therefore each new offspring, has a unique genetic inheritance.

Meiosis differs from normal cell division, or mitosis, in that it involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division. Whereas mitosis produces identical daughter cells, meiosis randomly mixes the chromosomes, resulting in unique combinations of chromosomes in each daughter cell.

Prior to meiosis, the corn cell undergoes Interphase, in which it synthesizes materials needed for cell growth and prepares for cell division. During this stage the cell’s genetic information, in the form of deoxyribonucleic acid (DNA), is replicated. Each of the two consecutive cell divisions consists of four stages: prophase, metaphase, anaphase, and telophase.

Prophase I each long DNA strand wraps around proteins that in turn coil and condense to form a chromosome. Since the DNA was copied during interphase, each chromosome condenses to form two identical chromatids, joined at a centromere. A corn cell has 20 chromosomes at this stage, each with two identical chromatids, making a total of 40 chromatids.

Chromosomes exist in pairs; one is inherited from the mother (maternal) and one from the father (paternal). When the chromosomes duplicate, two maternal and two paternal chromatids are produced. These two pairs of chromatids gather together in groups of four called tetrads. Each corn cell contains 10 tetrads. While grouped together in tetrads, sections of the chromatids from the maternal pair may randomly exchange, or cross over, with sections of the paternal chromatid pair. Called genetic recombination, this process is the first of two ways that meiosis mixes genetic information during sexual reproduction.

Also in Prophase I, two structures called centrioles, both located on one side of the nucleus, separate and move toward opposite sides of the cell. As the centrioles move apart, they radiate thin hollow structures called spindle fibers. The membrane around the nucleus of the cell breaks down, marking the beginning of the next stage.

During metaphase I, the spindle fibers attach to the chromatids near the centrioles. The spindle fibers move the tetrads so that they line up in a plane halfway between two centrioles.

Anaphase I begins when the spindle fibers pull the tetrads apart, pulling the maternal and paternal chromosomes toward opposite sides of the cell. The first meiotic division concludes with Telophase I, when the two new groups of chromosomes reach opposite sides of the cell. A nuclear membrane may form around the two new groups of chromosomes and a division of cell cytoplasm forms two new daughter cells.

Each daughter corn cell receives 10 chromosomes made up of a random mixture of maternal and paternal chromosomes. This second mixing of genetic information is called independent assortment. Genetic recombination and independent assortment make it possible for parents to have many offspring who are all different from each other.

In the second meiotic division the cell moves directly into prophase II, skipping the interphase replication of DNA. Each corn cell begins the second division with 10 chromosomes. Once again the centrioles radiate spindle fibers as they move to opposite sides of the cell. During metaphase II, the chromosomes line up along the plane in the center of the cell, and in anaphase II the pairs of chromatids are pulled apart, each moving toward opposite ends of the cell.

Telophase II completes meiosis. The spindle fibers disappear and a new nuclear membrane forms around each new group of chromosomes to form four haploid cells. The original diploid corn cell with 20 chromosomes has undergone meiosis to form four haploid daughter cells, each containing 10 chromatids. It is now possible for two haploid sex cells to join during fertilization to form one egg cell with the normal diploid number of chromatids. After fusion and DNA replication, two haploid corn cells will yield one diploid egg cell with 10 pairs of chromosomes.

In humans meiosis occurs only in the reproductive organs, the testes in males and the ovaries in females. In males, each of the meiotic divisions result in four equally sized haploid cells that mature into functional sperm cells. In females, the meiotic divisions are uneven, resulting in three tiny cells called polar bodies and one large egg that can be fertilized.

cell

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Cell

Cell (biology), basic unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell. Plants, animals, and fungi are multicellular; that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of trillions in a human being, the cell is a marvel of design and efficiency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life.

Prokaryotic Cells

Prokaryotic cells have no distinct nucleus. These  cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. About a hundred typical prokaryotic cells lined up in a row would match the thickness of a book page. These cells, which can be rodlike, spherical, or spiral in shape, are surrounded by a protective cell wall. Like most cells, prokaryotic cells live in a watery environment, whether it is soil moisture, a pond, or the fluid surrounding cells in the human body. Tiny pores in the cell wall enable water and the substances dissolved in it, such as oxygen, to flow into the cell; these pores also allow wastes to flow out.

Eukaryotic Animal Cells

The eukaryotic cell cytoplasm is similar to that of the prokaryote cell except for one major difference: Eukaryotic cells house a nucleus and numerous other membrane-enclosed organelles. Like separate rooms of a house, these organelles enable specialized functions to be carried out efficiently. The building of proteins and lipids, for example, takes place in separate organelles where specialized enzymes geared for each job are located.

Eukaryotic Plant Cells

Plant cells have all the components of animal cells and boast several added features, including chloroplasts, a central vacuole, and a cell wall. Chloroplasts convert light energy—typically from the Sun—into the sugar glucose, a form of chemical energy, in a process known as photosynthesis. Chloroplasts, like mitochondria, possess a circular chromosome and prokaryote-like ribosomes, which manufacture the proteins that the chloroplasts typically need.

Cell Functions

To stay alive, cells must be able to carry out a variety of functions. Some cells must be able to move, and most cells must be able to divide. All cells must maintain the right concentration of chemicals in their cytoplasm, ingest food and use it for energy, recycle molecules, expel wastes, and construct proteins. Cells must also be able to respond to changes in their environment.

Movement of  Prokaryotes

Many unicellular organisms swim, glide, thrash, or crawl to search for food and escape enemies. Swimming organisms often move by means of a flagellum, a long tail-like structure made of protein. Many bacteria, for example, have one, two, or many flagella that rotate like propellers to drive the organism along. Some single-celled eukaryotic organisms, such as euglena, also have a flagellum, but it is longer and thicker than the prokaryotic flagellum. The eukaryotic flagellum works by waving up and down like a whip. In higher animals, the sperm cell uses a flagellum to swim toward the female egg for fertilization.

Movement of Eukaryotes

Movement in eukaryotes is also accomplished with cilia, short, hair like proteins built by centrioles, which are barrel-shaped structures located in the cytoplasm that assemble and break down protein filaments. Typically, thousands of cilia extend through the plasma membrane and cover the surface of the cell, giving it a dense, hairy appearance. By beating its cilia as if they were oars, an organism such as the paramecium propels itself through its watery environment. In cells that do not move, cilia are used for other purposes. In the respiratory tract of humans, for example, millions of ciliated cells prevent inhaled dust, smog, and micro organisms from entering the lungs by sweeping them up on a current of mucus into the throat, where they are swallowed. Eukaryotic flagella and cilia are formed from basal bodies, small protein structures located just inside the plasma membrane. Basal bodies also help to anchor flagella and cilia.

Nutrition

All cells require nutrients for energy, and they display a variety of methods for ingesting them. Simple nutrients dissolved in pond water, for example, can be carried through the plasma membrane of pond-dwelling organisms via a series of molecular pumps. In humans, the cavity of the small intestine contains the nutrients from digested food, and cells that form the walls of the intestine use similar pumps to pull amino acids and other nutrients from the cavity into the bloodstream. Certain unicellular organisms, such as amoebas, are also capable of reaching out and grabbing food. They use a process known as endocytosis, in which the plasma membrane surrounds and engulfs the food particle, enclosing it in a sac, called a vesicle, that is within the amoeba’s interior.

Energy

Cells require energy for a variety of functions, including moving, building up and breaking down molecules, and transporting substances across the plasma membrane. Nutrients contains energy, but cells must convert the energy locked in nutrients to another form—specifically, the ATP molecule, the cell’s energy battery—before it is useful. In single-celled eukaryotic organisms, such as the paramecium, and in multicellular eukaryotic organisms, such as plants, animals, and fungi, mitochondria are responsible for this task. The interior of each mitochondrion consists of an inner membrane that is folded into a mazelike arrangement of separate compartments called cristae. Within the cristae, enzymes form an assembly line where the energy in glucose and other energy-rich nutrients is harnessed to build ATP; thousands of ATP molecules are constructed each second in a typical cell. In most eukaryotic cells, this process requires oxygen and is known as aerobic respiration.

Protein Synthesis

A typical cell must have on hand about 30,000 proteins at any one time. Many of these proteins are enzymes needed to construct the major molecules used by cells—carbohydrates, lipids, proteins, and nucleic acids—or to aid in the breakdown of such molecules after they have worn out. Other proteins are part of the cell’s structure—the plasma membrane and ribosomes, for example. In animals, proteins also function as hormones and antibodies, and they function like delivery trucks to transport other molecules around the body. Hemoglobin, for example, is a protein that transports oxygen in red blood cells. The cell’s demand for proteins never ceases.

Before a protein can be made, however, the molecular directions to build it must be extracted from one or more genes. In humans, for example, one gene holds the information for the protein insulin, the hormone that cells need to import glucose from the bloodstream, while at least two genes hold the information for collagen, the protein that imparts strength to skin, tendons, and ligaments. The process of building proteins begins when enzymes, in response to a signal from the cell, bind to the gene that carries the code for the required protein, or part of the protein. The enzymes transfer the code to a new molecule called messenger RNA, which carries the code from the nucleus to the cytoplasm. This enables the original genetic code to remain safe in the nucleus, with messenger RNA delivering small bits and pieces of information from the DNA to the cytoplasm as needed. Depending on the cell type, hundreds or even thousands of molecules of messenger RNA are produced each minute.