A Closer Look at Biology, Microbiology, And the Cell

Published in 2012 by Britannica Educational Publishing(a trademark of Encyclopdia Britannica, Inc.)in association with Rosen Educational Services, LLC29 East 21st Street, New York, NY 10010.Copyright 2012 Encyclopdia Britannica, Inc. Britannica, Encyclopdia Britannica, and theThistle logo are registered trademarks of Encyclopdia Britannica, Inc. All rights reserved.Rosen Educational Services materials copyright 2012 Rosen Educational Services, LLC.All rights reserved.Distributed exclusively by Rosen Educational Services.For a listing of additional Britannica Educational Publishing titles, call toll free (800) 237-9932.First EditionBritannica Educational PublishingMichael I. Levy: Executive Editor, Encyclopdia BritannicaJ.E. Luebering: Director, Core Reference Group, Encyclopdia BritannicaAdam Augustyn: Assistant Manager, Encyclopdia BritannicaAnthony L. Green: Editor, Comptons by BritannicaMichael Anderson: Senior Editor, Comptons by BritannicaSherman Hollar: Associate Editor, Comptons by BritannicaMarilyn L. Barton: Senior Coordinator, Production ControlSteven Bosco: Director, Editorial TechnologiesLisa S. Braucher: Senior Producer and Data EditorYvette Charboneau: Senior Copy EditorKathy Nakamura: Manager, Media AcquisitionRosen Educational ServicesHeather M. Moore Niver: EditorNelson S: Art DirectorCindy Reiman: Photography ManagerKaren Huang: Photo ResearcherMatthew Cauli: Designer, Cover DesignIntroduction by Heather M. Moore NiverLibrary of Congress Cataloging-in-Publication DataA closer look at biology, microbiology, and the cell / edited by Sherman Hollar.1st ed.p. cm.(The Environment: Ours to Save)In association with Britannica Educational Publishing, Rosen Educational Services.Includes bibliographical references and index.ISBN 978-1-61530-563-6 (eBook)1. BiologyJuvenile literature. 2. MicrobiologyJuvenile literature. 3. CellsJuvenile literature. I.Hollar, Sherman.QH309.2C56 2011570dc222011000401On the cover (front and back), page 3: Red blood cells. Shutterstock.comOn the front cover: Hand holding test tubes. Shutterstock.comPages 18, 21, 22, 32, 33, 46, 55, 83, 84, 87, 90, 93, 94 www.istockphoto.com/ChristianAnthony; remaining interior background images www.istockphoto.com/Henrik Jonsson

C ON T E N T S

Introduction

Chapter 1Areas of Study in Biology

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Chapter 2History of Biology

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Chapter 3The Field of Microbiology

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Chapter 4Cell Structure and Function

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Chapter 5History of Cell Theory

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ConclusionGlossaryFor More InformationBibliographyIndex

INTRODUCTION

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lants, animals, fungi, protozoa, algae,bacteria, and viruses all inhabit thenatural world. Biology is the studyof these and other living things. Todaysconstantly advancing technology allowsresearchers to investigate natures tiniest living organisms; this field of study is known asmicrobiology.In this volume you will learn about manyof the branches of biology. The sheer volume of scientific information available canbe mind-boggling, but areas of specializationallow scientists to focus on certain areas, likeanimals (zoology) or plants (botany). Somebiologists study even more specific areas,such as insects (entomology) or bacteria(bacteriology).You will also learn about the history ofbiology. The early Greeks were the first toformally study the natural world. Duringthe Renaissance, Leonardo da Vinci linkedhuman anatomy to that of animals. Swedishbiologist Carolus Linnaeus devised the modern method of classifying organisms, knownas taxonomy.The development of the microscopehas been and continues to be a huge scientific advancement. Scientists began to

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Introduction

develop microscopes as early as the 1600s.This powerful tool allowed them to study allkinds of previously unknown processes andstructures, including the cell. Modern microscopes, in particular electron microscopes,have helped scientists unravel the mysteriesof DNA.Areas of microbiology you will read aboutinclude bacteriology, protozoology (the studyof protozoans), phycology (algae), mycology(fungi), virology (viruses), and exobiology(life outside Earth). Some microbiologistshave made great strides with pure cultures(cultures containing the growth of a singlekind of organism free from other organisms)and methods of cultivating and identifyingmicrobes.You will also learn about the very unitthat interests so many scientists: the cell.Some organisms, such as yeasts, are singlecelled, but others, such as humans, arecomposed of many billions of cells. Simplecells only have a few parts, but more complex cells have a variety of parts with manyfunctions.Finally, you will learn about the richhistory of cell theory. Robert Hookefirst coined the term cell in the 1600s.

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A Closer Look at Biology, Microbiology, and the Cell

Cells reproduce through a process of division called mitosis. Paul Zahl/National Geographic Image Collection/Getty Images

Scientists initially made slow progress intheir studies and observations of cells.Anthony van Leeuwenhoek gave the firstaccurate description of red blood cells, butit was not until the 1800swhen bettermicroscopes were availablethat biologistscould see details that proved cells containliving material.

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Introduction

Scientists also investigated the origin ofcells. The theory of free cell formation, inwhich it was thought that cells developedfrom an unformed substance, persisted formany years. Eventually, Rudolph Virchowaffirmed that all cells come from cells, butthe complexities of cell division gave biologists pause. Walther Flemmings methods offixing and staining cells revealed how chromosomes move from the parent to daughtercells by the process of mitosis. Subsequently,the discovery that the number of chromosomes remains constant from one generationto the next led to the full description of theprocess of meiosis.Biologists continue to question how lifearound us occurs. The fields of biology andmicrobiology have produced many important discoveries, including the medicines andantibiotics that treat infections and illnessesevery day. Much about the natural worldremains a mystery, but persistent studies inthese fields will lead toward a better understanding of living things and the environmentthat supports them.

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C hapter 1Areas of Study in Biology

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he scientific study of living thingsis called biology. Biologists striveto understand the natural worldand its living inhabitantsplants, animals,fungi, protozoa, algae, bacteria, archaea, andvirusesby asking why and how the processesof life occur. Why do living organisms interact with each other in particular ways? Whendid they evolve? How are biological processescarried out within organs, tissues, and cells?To answer these broad questions, biologistsmust answer many specific ones: How doesan animals liver break down fat? How does agreen plant convert water and carbon dioxide into sugar? Where do mosquitoes go inthe winter?Some investigations require years of scientific research. Today many mysteries remainunsolved, but continued study leads toward abetter understanding of living things and theenvironment they depend on.The annual output of biological researchtoday is so massive that no single individualcan possibly acquire all of the information.Because of this, areas of specialization have

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Areas of Study in Biology

Some biologists specialize in a single area, such as zoology. HemeraTechnologies/AbleStock.com/Thinkstock

developed, allowing scientists to focus ontheir own research, yet remain informed onkey developments in their fields. Some biologists focus their research onone or several groups of organisms. Such specializations can be broad, such as zoology(the study of animals) and botany (the studyof plants); or they can be specific, as in thefollowing fields:

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Arachnology: spiders, mites,scorpions Bryology: mosses Entomology: insects Herpetology: reptiles and amphibians Ichthyology: fishes Mammalogy: mammals Microbiology: microscopicorganisms Mycology: fungi Ornithology: birds Parasitology: parasites Phycology: algae Virology: virusesSome biologists study specific features,such as structure, or explore broad biologicalconcepts. Such studies often look for generalprinciples that apply to different types oforganisms. Some examples are: Anatomy: the structure of livingthings Cytology: cells Ethology: animal behavior Genetics: heredity Pathology: disease and its effect onthe body Physiology: biological functions

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Biologists may be identified by thegroup of organisms they study or by theirarea of research. For example, a scientistwho studies nonhuman primates (such asapes and monkeys) is called a primatologist;a scientist who studies genetics is called ageneticist.The following sections discuss a smallfraction of the many specialized areas of biological research.

TaxonomyNaming organisms and establishing theirrelationships to one another comprise thefield of taxonomy (also called systematics). Modern taxonomy is based on a systemestablished in the 1750s by Swedish botanistCarolus Linnaeus.The Linnaean system classifies organismsbased on shared attributes and the closenessof their evolutionary relationships. The mostbasic category is the species (spelled identically for both singular and plural forms).Individual members of a species share common characteristics and a closer geneticrelationship with each other than they sharewith members of other species. The nexthighest taxon (level of organization) is the

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Carolus Linnaeus. Hulton Archive/Getty Images

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genus (plural: genera), which includes groupsof related species.All species have a two-part scientificname. The first part is the genus, or generic,name. For example, wolves and coyotesbelong to the same genus: Canis. The secondpart of the name is the specific name: wolvesare members of the species Canis lupus, whilecoyotes belong to the species Canis latrans.The whole scientific name is always italicized; the generic name is capitalized, whilethe specific name is not.The relatedness between groups withina taxon becomes increasingly distant athigher levels: genera with similar traits aregrouped into the same family; relatedfamilies are classified in the same order;related orders are placed into the sameclass; related classes are placed in the samephylum; related phyla (plural of phylum)are placed into a kingdom; and relatedkingdoms are placed into a domain, thehighest level of classification. The highertaxonomic levels indicate phylogeneticrelationshipsthe degree to which specieshave diverged from each other during thecourse of evolution.The classification of living things is frequently challenged and revised. Taxonomic

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studies may be based on morphological(structural) traits, such as skull shape and jawlength, or on molecular data, such as DNA,RNA, or protein sequences.

Embryology andDevelopmental BiologyDevelopmental biologists examine theprocesses that control the growth anddevelopment of organisms. Includedwithin the field are studies of embryological development of plants or animals andthe natural phenomenon of regeneration inwhich removed cells, tissues, or entire structures of an organism grow back. Researchin development has direct applications foragriculture and for human and veterinarymedicine. An example of this is cloning,in which cells from an adult plant or animal are used to grow a genetically identicalindividual. Plant cloning is widely used inagriculture and horticulture. Several typesof animals, such as sheep, cows, and cats,have been cloned, though the practice is notwidespread. Stem cell research is anotherexample of developmental biology. Thecapability of stem cells (cells in extremely

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Although cloning and stem cell research show a great deal of medical promise, their use remains controversial. Peter Dazeley/Photographers Choice/Getty Images

early stages of development) to grow manydifferent kinds of living tissue in laboratorycultures has broad potential in medicine.Despite their potential in medicine, however, cloning and stem cell research remaincontroversial.

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BioethicsBiology and medicine are sciences, and theyboth deal with living beings. They have directeffects on human beings and other living species, so they quickly raise ethical and other valueproblems as well as scientific ones. Bioethics isthe branch of ethics, or moral decision-making,that deals with the problems of biology andmedicine. It requires disciplined, systematicreflection on these difficult issues.Scientists can change the genetic information in bacteria and are rapidly developing thecapacity to change it in many animal species,including humans. But should they? Peoplechange the nature of the human populationby aborting defective or unwanted fetuses,controlling when pregnancy occurs, and planning limits on population size. But shouldthey? Physicians can keep seriously ill patientsalive indefinitely, using artificial respirators,machines that take over the control of thebeating of the heart, and drugs to control bloodpressure and consciousness. But should they?People are beginning to ask whether therecomes a time when patients should be allowedto die. Citizens are claiming patients rights,insisting on being informed about medical procedures and deciding how to allocate healthresources fairly. When they ask these questionsand make these decisions, they are dealing withbioethics.

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Anatomy andMorphologyAnatomists study the structure of organisms.Some morphological research compareshomologous (similar in origin) or analogous(similar in function) structures among different species to establish phylogeneticrelationships. Other studies may investigatethe function or mode of operation of an anatomical feature. Histology (study of tissues)and cytology (study of cells) are specializedareas of morphology.

PhysiologyA physiologist studies the functions oforgans and tissues. A cell physiologistinvestigates processes at the subcellularlevel. Animal or plant physiologists maystudy entire systems, such as circulatory orrespiratory. Many physiological studies areintimately associated with morphology.

Genetics andMolecular BiologyMolecular biology and genetics are two ofthe most dynamic fields of biology today.

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New laboratory techniques developed during the 20th century allowed scientiststo examine the structure and function ofbiological molecules, such as DNA and proteins, and determine their relationship tocellular structures, such as the nucleus andcell membrane. Geneticists also have benefited from molecular studies on genes andchromosomes. However, the use of geneticengineering in medicine and agriculturehas raised many new moral and philosophical issues.

DNA contains two strands of nucleotides linked together by chemical bonds. Each nucleotide contains a phosphate, deoxyribose (asugar), and one of four nitrogen-containing basesadenine (A),guanine (G), cytosine (C), or thymine (T). The bases of one strandare linked to bases of the second strand. Because of their structures,adenine can only pair with thymine, and cytosine can only pair withguanine. Encyclopdia Britannica, Inc.

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Genetic EngineeringAlmost every living cell holds a vast storehouseof information encoded in genes, segmentsof DNA that control how the cell replicatesand functions and the expression of inheritedtraits. The artificial manipulation of one ormore genes to modify an organism is calledgenetic engineering.The term genetic engineering initially encompassed all the methods used formodifying organisms through heredity andreproduction. These included selective breeding, or artificial selection, as well as a widerange of biomedical techniques such as artificial insemination, in vitro fertilization, andgene manipulation. Today, however, the term isused to refer to the latter technique, specificallythe field of recombinant DNA technology. Inthis process DNA molecules from two or moresources are combined and then inserted into ahost organism, such as a bacterium. Inside thehost cell the inserted, or foreign, DNA replicates and functions along with the host DNA.Recombinant DNA technology has produced many new genetic combinations thathave greatly affected science, medicine, agriculture, and industry. Despite the tremendousadvances afforded to society through this technology, however, the practice is not withoutcontroversy. Special concern has been focused

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on the use of microorganisms in recombinanttechnology, with the worry that some geneticchanges could introduce unfavorable andpossibly dangerous traits, such as antibioticresistance or toxin production, into microbesthat were previously free of these.

EcologyEcologists study the relationships andinteractions between organisms and theirenvironments by examining the structureand function of ecosystems. Many ecological studies require input from otherscientific disciplines, such as geology, animal behavior, and botany. Policy makersand scientists interested in conservationissues need a solid understanding of ecology to understand how changes such aspollution and habitat destruction affectnatural communities at both the local andthe global level.

Ethology and SociobiologyEthologists, or animal behaviorists, attemptto understand why animals behave theway they do. Some studies involve direct

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Ethologist Jane Goodall. Fotos International/Archive Photos/Getty Images

observations of animals in their native habitats, while others may involve experimentsusing laboratory animals. Ethology is tiedclosely to the fields of psychology and sociology. Sociobiology is concerned with thesocial interactions within a given species andfocuses on such issues as whether certaintraits, such as intelligence, are inherited orare culturally induced.

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Paleontology (the study of fossils) can be a useful tool for many evolutionary biologists. Photodisc/Thinkstock

Evolutionary BiologyThe evolution of species by natural selectionis considered by the great majority of biologists to be a fundamental tenet of modernbiology. Evolutionary biology seeks to answerquestions about the origin and the geneticrelationships of all living things. Someevolutionary biologists examine genetic relationships by comparing DNA sequences,

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while others may compare structural featuresor physiology. Many evolutionary biologistsuse knowledge gleaned from paleontology(the study of fossils).

Other Areas of StudyAlthough the aforementioned categoriesrepresent the major subdivisions of biology,there are many other research areas. Someare based on life in specific environments.Marine biology, for example, looks at manyaspects of ocean life, whereas soil biologyfocuses on organisms and processes occurring in soil.Many other scientific disciplines alsorequire knowledge of biology. For example,biochemistry, a subdivision of organicchemistry, focuses on subcellular chemicalprocesses and requires a solid foundation incell biology.

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C hapter 2History of Biology

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o one knows precisely whenhumans first began to acquireknowledge of the natural world.Most experts believe that humans had beendomesticating many animals and cultivatingcrops long before written records were kept.The earliest records show that the Assyriansand Babylonians had some knowledge of agriculture and medicine as early as 3500 bc. By2500 bc this knowledge was widely appliedby the major civilizations of China, Egypt,and India.

The Greeks and Natural LawThe early Greeks were the first to formallyinvestigate and describe the natural world.The concepts of cause and effect and thatof a natural law that governs the universewere proposed around 600 bc. Some 200years later, the Greek physician Hippocratesobserved among other things the effect ofthe environment on human nature.In the mid-4th century bc Aristotle presented the first system for classifying animals

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History of Biology

Aristotle. Photos.com/Thinkstock

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based on similarity of structure and function.His student Theophrastus drew up a schemefor classifying many of the plants. The writings of Galen, a Greek physician who lived inRome during the 2nd century ad, influencedmedicine for hundreds of years.

The Middle AgesDuring the Middle Ages (roughly ad 5001400), the center of biological studies shiftedfrom Europe to the Middle East. The Islamicscholar al-Jahiz expanded on the observations of the Greeks. His multi-volume Bookof Animals discussed a variety of topics, suchas the relationships among different animalgroups and animal mimicry. The writings ofthe Persian physician Avicenna (Ibn Sina),based on the observations of Aristotle,helped revive European interest in biology.

A Rebirth of ScientificLearningMajor biological advancements were madein Europe during the Renaissance (about ad1300 to 1650). The serious study of anatomyemerged in the 1500s through the efforts ofLeonardo da Vinci and Andreas Vesalius, who

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History of Biology

documented the relationships between theanatomies of humans and of other animals.Advances in anatomy and physiology weremade by means of dissection of organismsduring the 16th and 17th centuries.Prior to the 16th century, it was commonly believed that organisms such asflies and worms arose from mud or otherinanimate substrates. Although some scientists had previously disputed this idea ofspontaneous generation,theconcept remaineduntested. In 1668the Italian physician FrancescoRedi was the firstto challenge theconcept using aset of controlledexperiments.Interest in botany also increasedduring the 16thand 17th centuries.

Francesco Redi. De Agostini Picture Library/Getty Images

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Numerous papers published by botanistssuch as Otto Brunfels of Germany andGaspard Bauhin of Switzerland discussedhorticulture and other plant-related topics.

Replica of Robert Hookes compound microscope. The 17th-centuryinvention and development of the microscope revealed unknown processes and organisms. Dave King/Dorling Kindersley/Getty Images

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History of Biology

Development ofthe MicroscopeThe invention and development of themicroscope in the 1600s generated anexplosion of interest in biological studies.The value of this important new researchtool was phenomenal. Unsuspected processes and organisms unknown to sciencewere discovered in a flurry of biologicalinvestigation. Anthony van Leeuwenhoekreported his observations of single-celledanimal-like creatures (protozoa) invisible to the naked eye. He subsequentlyobserved spermatozoa, leading to newquestions and interpretations of the malerole in fertilization and reproduction.

Onion cells. The microscope enabled Robert Hooke to observe tinycompartments he called cells. Shutterstock.com

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Anthony vanLeeuwenhoekBy means of his extraordinary ability to grindlenses, Anthony van Leeuwenhoek greatlyimproved the microscope as a scientific tool.This led to his doing a vast amount of innovative research on bacteria, protozoa, and othersmall life-forms that he called animalcules(tiny animals).Leeuwenhoek was born in Delft, Holland,on Oct. 24, 1632. He probably did not have muchscientific education because his family couldnot afford it. He first became a haberdasher (adealer in mens clothing and accessories) anddraper (a dealer in cloth or clothing and drygoods) and, in 1660, chamberlain to the sheriffs at Delft. His hobby was lens grinding. Inhis lifetime he ground about 400 lenses, mostof which were quite small, with a magnifyingpower of from 50 to 300 times.It was not only his lenses that made himworld famous but also his work with the microscope. His keen powers of observation led todiscoveries of major significance. For example,he observed and calculated the sizes of bacteria and protozoa and gave the first accuratedescription of red blood cells.Although Leeuwenhoek lived in Delft, hemaintained a regular correspondence withthe Royal Society of England, to which he

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History of Biology

was elected in 1680. Most of his discoverieswere published in the societys PhilosophicalTransactions. He continued his work throughout most of his 90 years. He died in Delft onAug. 26, 1723.

The concept of cells was introduced in 1665,when the English physicist Robert Hookereported on the presence of tiny compartments in tissue he was studying under amicroscope. Hooke named the compartmentscells. Marcello Malpighi used the microscopeto observe and describe many microscopicstructures, including red blood cells. Manyother contributions to biology were made during this period as a result of discoveries in thispreviously unseen microscopic world.

Biological ClassificationThe publication in the 1750s of CarolusLinnaeus biological classification schemefor organisms was a major advance in biology.Linnaeus was one of the first taxonomists toorganize living things in a simple and logicalmanner, using a system of binomial nomenclature (two-part names) that appealed to mostscientists. The Linnaean system indicates

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both the degree of similarity and differenceamong species, and it persists today as thebasis for naming living things.

Evolutionary TheoryNew biological theories developed rapidlyduring the 18th and 19th centuries and challenged many old ideas. The British naturalistCharles Darwin published his theory aboutevolution in the book On the Origin of Speciesby Means of Natural Selection (1859). Darwinsideas centered around observations he hadmade in the Galpagos Islands, an archipelago off the coast of Ecuador. AnotherBritish naturalist, Alfred Russel Wallace,made similar observations about animals inIndonesia, and the research of both scientistswas presented simultaneously to their peers.Although Darwins efforts received widerattention, Wallaces observations about thegeographic distribution of plants and animalsremain vital in modern studies of evolution.The concept of natural selection and evolution revolutionized 19th-century thinkingabout the relationships between groups ofplants and animals and about speciation (theorigin of new species). Darwin provided soundscientific reasoning for the wealth of biological

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History of Biology

variability and similarity that exists amongliving things. Although genetics and the mechanisms of inheritance were unknown duringDarwins time, he noted that certain life formswere more likely to survive than others, andproposed that this was influenced by variabletraits (such as beak size in birds) that werepassed from parents to offspring. This conceptof natural selection provided the first scientific explanation of the variations observed innature. Darwin also proposed that new speciesare formedand others become extinctbya gradual process of change and adaptationmade possible by this natural variability.Although Darwins ideas provoked tremendous controversy, they influenced biology morethan any other concept and today are generallyaccepted by the scientific community.

Mechanism of HeredityThe mechanism that produced the heritablevariation needed for natural selection wasdiscovered in the mid-19th century by GregorMendel. An Austrian monk interested inplant breeding, Mendels experiments withgarden peas revealed that the peas inheritedcharacteristics from their parents in a mathematically predictable fashion. His findings

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Gregor Mendel studied the blossoms of garden peas and proved themathematical foundation of the science of genetics, in what came to becalled Mendelism. Shutterstock.com

introduced the concept of the gene as the unitof inheritance, or heredity. Although Mendelpublished his results in 1866, the significanceof his studies remained obscure until 1900.The rediscovery of Mendels work and thediscovery of chromosomes in the early 20thcentury spurred development of studies ofgenetics and heredity and strengthened sciences understanding of evolution.36

History of Biology

Modern DevelopmentsAn important milestone in the history ofbiology was the discovery in 1953 of the structure of DNA and the subsequent unravelingof the genetic code of life. These discoveriesaided sciences understanding of genetic diseases in plants and animals and allowed forunprecedented discoveries in molecular biology. Advances in the technology for copyingand manipulating DNA ushered in the ageof biotechnology with practical applicationsin agriculture, industry, and medicine. It alsoenabled efforts to decipher the entire geneticcode (genome) of many organisms. As geneticsequencing became faster and less expensive,it spurred biological research in such areas asthe study of gene expression and function inbiological processes.Some developments had negative effectson the natural world, however. Increasedurbanization and industrialization destroyedmany habitats and threatened the existenceof countless species, while pollution and theemergence of new infectious diseases suchas AIDS endangered public health. Thegrowth of biotechnology also raised concerns over its potential hazards to healthand the environment and the need to monitor and regulate its use.37

C hapter 3The Field of Microbiology

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cientific exploration to understand thenature of the tiniest living organismsconstitutes the field of microbiology.Such organisms are known as microbes, andthe scientists who study them are calledmicrobiologists.Over the years, microbiology has extendedto include more than microbes alone. Forinstance, the field of immunology, whichstudies the bodys reaction to microbes, isclosely aligned with microbiology. In addition, a whole new field of molecular biologyhas emerged. Today molecular biologistsstudy the properties of cellular structuressuch as proteins and nucleic acids.Microbes are widely spread over the surface of the Earth and play a crucial role inecology. Soil and water contain high concentrations of bacteria and molds (two types ofmicrobes), and the surface of every humanbody is covered with a unique microbial flora.Certain bacteria draw nitrogen from the airand pass it on to plants in the soil. Othershelp break down and recycle organic materials and waste products.

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The Field of Microbiology

The action caused by yeast microbes makes bread dough rise.Shutterstock.com

The action of microbes has also been harnessed for industrial uses. Yeast is used inthe production of bread and alcohol. Othermicroscopic organisms are used for theproduction of many foodstuffs and for thedegradation of industrial by-products. Theresearch and development of microbes forsuch practical uses is the subject of appliedmicrobiology.

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Areas of StudyMicrobiologists classify microorganismsinto bacteria, protozoans, algae, fungi, andviruses, and the study of each constitutesa separate specialty within microbiology.Individual fields may overlap, and the discipline of microbiology may overlap with otherdisciplines, as it does with immunology. Inany case, most areas of scientific inquiry canbe subdivided in a variety of ways, dependingon the questions being asked. Microbiologymay be subdivided as follows.

BacteriologyThe study of bacteria is called bacteriology.Bacteria are single-cell microbes that growin nearly every environment on Earth. Theyare used to study disease and produce antibiotics, to ferment foods, to make chemicalsolvents, and in many other applications.

ProtozoologyProtozoology is the study of protozoans,small single-cell microbes. They are frequently observed as actively moving

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organisms when impure water is viewedunder a microscope. Protozoans cause anumber of widespread human illnesses, suchas malaria, and thus can present a threat topublic health.

Phycology and MycologyPhycologists study algae. In general, algae areorganisms that are made up of one or moreeukaryotic cells (cells with a true nucleus)that contain chlorophyll and that are lesscomplex than plants. Mycology is the studyof fungiwell-known organisms that lackchlorophyll, as well as the organized plantstructures of stems, roots, and leaves. Fungiusually derive food and energy from parasiticgrowth on dead organisms.

VirologyViruses are a vastly different kind of biological entity. They are the smallest formof replicating microbe. Viruses are neverfree-living; they must enter living cells togrow. Thus they are considered by mostmicrobiologists to be nonliving. There isan infectious virus for almost every known

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kind of cell. Viruses are visible only with themost powerful microscopes, namely electronmicroscopes.

ExobiologyExobiology is the study of life outside theEarth, including that on other planets. Spaceprobes have been sent to Mars, and samples

Exobiologists search for traces of microbial life in rock samples fromMars and other planets. NASA/JPL-Caltech/University of Arizona

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of rocks have been brought back from ourmoon as part of experiments to search fortraces of microbial life in extraterrestrialenvironments. Most exobiologists examinesuch samples for the basic building blocksof life known to have evolved on Earth.Because microbes were probably the earliestorganisms on Earth, and because they havecontinued to thrive, exobiologists considermicrobes to be the most likely form of life toexist beyond the Earth.

Biological WarfareAnother area of study in microbiologyinvolves the development, deployment, anddefense against agents of biological warfare.Both chemical and biological agents havebeen used in past wars because they are oftenmore insidious and less easily detected thanconventional weapons. This application ofmicrobiology has been made still more ominous by the ability to alter microbes usinggenetic engineering.

Methods of MicrobiologyThe chief tool of the microbiologist hasalways been the microscope. Since its

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invention there have been great refinementsin the optical microscopes power and precision. In addition, the electron microscopeand other high-energy devices allow microbiologists to view the smallest structures oflife, including the DNA molecule. However,because the powerful forms of energy thatare necessary for electron microscopes, suchas X-rays and particle beams, will destroybiological specimens, scientists have developed new technologies for viewing cells.The transmission electron microscope, forexample, produces a shadow of the specimen by evaporating platinum metal over theviewing platform at a sharp angle. When electrons are passed through the platform at highspeed, they can distinguish the shadows onthe metal as a representation of the originalspecimen. Scanning electron microscopes,however, view the surface of a specimen byreflected radiation. In this case, the sample isalso thinly coated with a heavy metal such asgold, and the biological material is observedas a cast of the more stable material. Thevarious types of electron microscopes haveallowed microbiologists to study in detail thefine structures of bacteria and viruses. Withcontinued refinements and the developmentof new technologies in microscopy, it may

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Electron microscopes give microbiologists the ability to view minuscule structures like chromosomes. Shutterstock.com

eventually prove possible to view individualgenes or protein molecules within a cell.Advances have also occurred in the use ofpure cultures, and improvements in methodsof growing and identifying microbes havefound wide application in all areas of microbiology. In the laboratory, scientists must havepure cultures of microbes for their studies.If contaminating organisms are present, the

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The Work ofMicrobiologistsProfessional microbiologists are employedin a wide variety of positions. The majoritywork in universities, government agencies, orindustry. In colleges and universities, microbiologists teach and conduct research. Stateand federal governments employ microbiologists to conduct research and to helpregulate private-sector activities. For example,in the United States some microbiologists areemployed by the federal government to inspectsewage-treatment facilities, hospitals, andfood-production plants to protect the public health. Large federal agencies, such as theNational Institutes of Health, the Departmentof Agriculture, and the Centers for DiseaseControl also employ microbiologists to investigate and monitor the action of microbes inour environment. There are also many commercial positions available to microbiologists.For example, wineries and breweries employmicrobiologists to help standardize and maintain yeast cultures. In many other areas ofindustrial food production, the expertise ofmicrobiologists is employed in quality controlto guard against spoilage and contamination.Microbiologists are also employed by pharmaceutical companies to help produce vaccinesand other drugs.

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results of their experiments may be uselessor misleading. For these reasons, microbiologists maintain pure cultures of the knownmicrobes and provide them to associates forexperimental use.

History of MicrobiologyMicrobiology began with the developmentof the microscope in the 17th and 18th centuries. By 1680 Anthony van Leeuwenhoekhad produced a simple hand-held devicethat allowed scientists to view a variety ofmicrobes in stagnant water and in scrapingsfrom teeth. In the late 1700s Edward Jennerconducted the first vaccinations, using cowpox virus to protect people against smallpox.Later an altered form of the rabies virus wasused to protect against the dreaded diseaserabies. Vaccines remain the major means ofprotection against most viral infections.Modern microbiology had its origins in the work of the French scientistLouis Pasteurconsidered the father ofmicrobiologywho developed methodsof culturing and identifying microbes.During the second half of the 19th century, he and his contemporary Robert Kochprovided final proof of the germ theory

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of disease. They also demonstrated thatmicrobes must be introduced or seededinto a sterile environment and could notarise spontaneously, as had been previouslybelieved. Pasteur was the first to proposethat microbes cause chemical changes asthey grow. Koch derived a central principleof modern microbiology, known as KochsPostulate, that determines whether a particular germ causes a given disease.Pasteur and his contemporaries developed pure culture methods for the growth ofmicrobes. By diluting mixtures of microbesin sterile solutions, they were able to obtaindroplets that contained a single microbe,which could then be grown on fresh, sterilemedia. In a separate procedure, they usedrapid, sequential passage of cultures so thatcertain specimens were able to outgrow others. Thus, for the first time, stable culturescontaining a single kind of microbe could beused to identify and study specific diseasecausing organisms.Another great advance in pure culturemethods came in the late 19th century,when microbiologists discovered that eachkind of microbe preferred a certain mediumfor optimal growth. Over the past century,microbiologists have made great progress in

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Alexander Fleming discovered that the mold Penicillium prevents thegrowth of bacteria. Hemera/Thinkstock

the preparation of selective media for thepurification and identification of most species of microbes.In 1929 Alexander Fleming observed thatmolds can produce a substance that prevents the growth of bacteria. His discovery,an antibiotic called penicillin, was later isolated and produced commercially to protectpeople against the harmful effects of certain

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Microbiologists were able to better study the nature of DNA by usingsimple microbes. Comstock/Thinkstock

microorganisms. Today several kinds of penicillin are synthesized from various species ofthe mold Penicillium and used for differenttherapeutic purposes. Many other antibioticshave been identified as well, and they remainthe major line of defense against infectiousbacterial diseases.

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In the 1940s microbiology expanded intothe fields of molecular biology and genetics.Viruses were found to be simple microbesthat could be studied quantitatively, andthey were used to study the nature of DNA.Microbiologists began to work inside cellsto study the molecular events governing thegrowth and development of organisms.In the early 1970s, genetic researchers discovered recombinant DNA. Scientists foundthat DNA could be removed from living cellsand spliced together in any combination.They were able to alter the genetic code dictating the entire structure and function ofcells, tissues, and organs.

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C hapter 4Cell Structure and Function

T

he cell is the smallest unit of livingmatter that can exist by itself. Someorganisms, such as bacteria, consist ofonly a single cell. Others, such as humans andoak trees, are made up of many billions of cells.Cells exist in a variety of shapes and sizes.Red blood cells are disk-shaped, and someskin cells resemble cubes. A single cell couldbe as large as a tennis ball or so small thatthousands would fit on the period at the endof this sentence. Regardless of size, however,every cell contains the components neededto maintain life. Cells normally functionwith great efficiency, though they are vulnerable to disease.Cell size is usually measured in microns.A micron is equal to about one millionth of ameter, and about 25,000 microns equal 1 inch.The smallest bacteria are about 0.2 micron indiameter. The diameter of the average humancell is roughly 10 microns, making it barelyvisible without a microscope.All cells contain cytoplasm, a substancemade up of water, proteins, and other

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Cell Structure and Function

Red blood cells are shaped like disks, but other cells come in a varietyof shapes and sizes. iStockphoto/Thinkstock

molecules surrounded by a membrane. Thecytoplasm of eukaryotic cells also containsnumerous kinds of bodies called organelles.Much of the cells work takes place in thecytoplasm.

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Some typical eukaryotic cells. Encyclopdia Britannica, Inc.

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Prokaryotes andEukaryotesBased on fundamental differences in theircell structure, living organisms can be dividedinto two major groups: prokaryotes andeukaryotes. Bacteria and archaea are prokaryotes. Animals, plants, fungi, and protistsare eukaryotes.Prokaryotic and eukaryotic cells aredistinguished by several key characteristics. Both cell types contain DNA as theirgenetic material. Prokaryotic DNA is singlestranded and circular, however, and it floatsfreely inside the cell. Eukaryotic DNA isdouble stranded and linear and is enclosedinside a membrane-bound structure calledthe nucleus. Eukaryotes also have otherspecialized membrane-bound structures(organelles) that do much of the cells work.Although prokaryotes lack organelles, theymust accomplish many similar vital tasks.This inability to delegate tasks makes prokaryotes less metabolically efficient thaneukaryotes.

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Cell MembraneCells can survive only in a liquid medium thatbrings in food and carries away waste. Forunicellular (single-celled) organisms, such asbacteria, algae, and protozoa, this fluid canbe an external body of water, such as a lake orstream. For multicellular (many-celled) organisms, however, the liquid medium is contained

Sap brings in food and carries away waste in plants and trees.iStockphoto/Thinkstock

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within the organism. In plants, for example, itis the sap. In animals it is the blood.The cell membrane is semipermeablethat is, some substances can pass through itbut others cannot. This characteristic enablesthe cell to admit or block substances fromthe surrounding fluid and enables the cell toexcrete waste products into its environment.The cell membrane is composed of twothin layers of phospholipid molecules studded with large proteins. Phospholipids arechemically related to fats and oils. Somemembrane proteins are structural. Othersform pores that function as gateways to allowor prevent the transport of substances acrossthe membrane.Substances pass through the cell membrane in several ways. Small unchargedmolecules, such as water, pass freely downtheir concentration gradient (from the side ofthe membrane where they are in higher concentration to the side of lower concentration).This movement is called diffusion. Othermaterials, such as ions (charged molecules),must be transported through channelsmembrane pores that are regulated bychemical signals from the cell. This facilitatedtransport requires energy for substances moving against a concentration gradient.

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Passive and Active TransportSubstances such as glucose or ions enter thecell through specific channels, traveling downtheir concentration gradient. Because theprocess does not require energy, it is calledpassive transport.Molecules moving against their concentration gradient must be escorted acrossthe cell membrane. This is called activetransport, and it requires the cell to spendenergy. Chemical signals in the cell tell themembrane channels when to start and whento stop the transport process.

Endocytosis and ExocytosisEndocytosis is a process used by cells to takein certain materials. The cell membraneforms a pocket around a substance in itsenvironment. The filled pocket breaks loosefrom the membrane, forming a bubblelike vacuole that drifts into the cytoplasm,where its contents are digested: the vacuole wall is broken down and the contentsare released into the cytoplasm. The process is called pinocytosis (pino- is fromthe Greek pinein, meaning to drink) when

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the material is dissolved in fluid and phagocytosis (phago- is from the Greek phagein,meaning to eat) when the cell ingestslarger, particulate matter, such as anothercell. The reverse process, exocytosis, is usedto remove material from the cell.

Cell WallVirtually all prokaryotes, as well as the cellsof plants, fungi, and some algae, have a cellwalla rigid structure that surrounds thecell membrane. Most cell walls are composedof polysaccharideslong chains of sugarmolecules linked by strong bonds. The cellwall helps maintain the cells shape and, inlarger organisms such as plants, enables it togrow upright. The cell wall also protects thecell against bursting under certain osmoticconditions.Plant cell walls, as well as those of greenalgae and some other protists, are mademostly of the polysaccharide cellulose. Insome plants, the cellulose is mixed with varying amounts of other polysaccharides, suchas lignin, an important component of treebark and wood. In some fungi the cell wallis composed of chitin, a polysaccharide that

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also forms the exoskeleton of many invertebrates such as insects and crabs. The bacterialcell wall is composed mostly of peptidoglycan, which is made up of polysaccharides andamino acids. The cell walls of the diatom (atiny one-celled organism) have a high concentration of silica, which gives them a glasslikeappearance.

CytoplasmWater is the largest component of cytoplasm. Depending on the cell and its needsand conditions, water concentration variesfrom about 65 percent to roughly 95 percent.Suspended in the water are various solidssuch as proteins, carbohydrates, fat droplets,and pigments. As such, cytoplasm is a colloidrather than simply a solid or a liquid.Changes in the concentration of solids produce an apparent streaming of thecytoplasm from place to place within thecell. When viewed through a microscope,membranes and fibrous structures aremore readily visible in the cytoplasm whenthe concentration of solids increases. Thisvisibility decreases as the solid contentdecreases.

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Organelles andTheir FunctionsCells are constantly working to stay alive.Food molecules are changed into materialneeded for energy, and substances needed forgrowth and repair are synthesized, or manufactured. In eukaryotic cells most of thesetasks take place inside membrane-boundbodies of the cytoplasm called organelles.According to the theory of endosymbiosis,certain organellesin particular plastidsand mitochondriaoriginated as small independent prokaryotic cells that invaded orwere engulfed by primitive eukaryotic cellsand formed an interdependent relationshipwith them.

PlastidsPlastids are found in plant and algae cellsthat use photosynthesis to manufacture andstore food. Chloroplasts, chromoplasts, andleucoplasts are the most common plastids.Photosynthesis takes place inside chloroplasts, which contain chlorophyll, a greenpigment that captures energy from the sunand uses it to make sugar. Chromoplasts,

61


Chromoplasts contain pigments, such as orange carotenes and yellowxanthophylls, that create the vibrant fall colors seen in many tree species. www.istockphoto.com/Tony Lomas

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most commonly found in fruits and flowerpetals, contain other pigments, such asthe orange carotenes, yellow xanthophylls,and red and blue anthocyanins. These pigments give fruits and flowers their colorsand produce the brilliant fall hues seen inmany tree species. Leucoplasts are colorlessand usually contain starch granules or othermaterials.All plastids have an inner and an outermembrane. The inner membrane is highlyimpermeable, while the outer is semipermeable. Plastids have their own DNA. It isdistinct from the DNA found in the cellsnucleus and is replicated and inherited independently. Plastids manufacture some oftheir own proteins but rely on the cells DNAand ribosomes to synthesize others.

MitochondriaOften called the powerhouses of the cell, thesausage-shaped mitochondria produce theenergy needed by the cell to function. Foodmolecules that pass into the cytoplasm aretaken into the mitochondria and oxidized, orburned, for energy. Like plastids, mitochondria have an inner and an outer membrane.

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Encyclopdia Britannica Online School Edition. CopyrightEncyclopdia Britannica, Inc.; rendering for this edition by RosenEducational Services

Also like plastids, although they have theirown DNA, they depend on the cells DNAfor certain proteins.

Endoplasmic Reticulumand RibosomesThe endoplasmic reticulum (ER), a network of membranous tubes and sacs, twiststhrough the cytoplasm from the cell membrane to the membrane surrounding thenucleus. Located along portions of the

64


The endoplasmicreticulum (ER)plays a major rolein the biosynthesis of proteins.Proteins that aresynthesized byribosomes on theER are transportedinto the Golgiapparatus for processing. Some ofthese proteins willbe secreted fromthe cell, others willbe inserted into theplasma membrane,and still otherswill be insertedinto lysosomes.EncyclopdiaBritannica, Inc.

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endoplasmic reticulum are ribosomes, tinybodies made of RNA that play a vital role inthe manufacture of proteins. Ribosomes arealso found scattered throughout the cytoplasm; distinct sets of ribosomes are foundin plastids and mitochondria.The portions of the endoplasmic reticulum that contain ribosomes are called roughendoplasmic reticulum (RER). Areas of thenetwork that do not contain ribosomes arecalled smooth endoplasmic reticulum (SER).The latter is predominant in cells involved inthe synthesis and metabolism of lipids andthe detoxification of some drugs.

Golgi ComplexThe Golgi complex, or Golgi apparatus, is amembranous structure composed of stacksof thin sacs. Newly made proteins and lipidsmove from the RER and SER, respectively, tothe Golgi complex. The materials are transported inside vesicles formed from the ERmembrane. At the Golgi complex, the vesicles fuse with the Golgi membrane and thecontents move inside the Golgis lumen, orcenter, where they are further modified andsubsequently stored. When the cell signals

66


that certain proteins are needed, the latterare packaged by the Golgi for exportpart of the Golgi membrane forms a vesiclethat then buds off, or breaks away, from thelarger apparatus. The vesicle may migrate tothe cell membrane and export its contentsvia exocytosis or it may travel to an intracellular location if its contents are needed bythe cell itself. Lipids are processed by thesame methods.


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VacuolesVacuoles drift through the cytoplasm and usually carry food molecules in solution. Vacuolesalso regulate the water content of somesingle-celled organisms. For example, whenan amoeba absorbs too much water, it formsa contractile vacuole against the membrane.The vacuole fills with water and then contractsto squeeze the excess liquid out of the cell.Vacuoles in cambium cells in plantsdevelop large central vacuoles that play a rolein building stalks and stems. If a cambiumcell is to become bark or wood, its membranegrows into the vacuole and deposits layers ofcell wall to increase stiffness. In cells thatbecome part of a vascular bundle that transmits sap, the vacuole becomes cylindrical anddevelops openings at each end that pass sapfrom cell to cell.

Lysosomes and PeroxisomesLysosomes are similar in appearance to vacuoles. Each lysosome is filled with enzymesthat help the cell to digest certain materials,such as cell parts that are no longer functional, and foreign particles, such as bacteria.

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Similar to lysosomes are peroxisomes, whichcontain enzymes that destroy toxic materialssuch as peroxide. Lysosomes are producedin the Golgi complex, while peroxisomes areself-replicating.

CentrosomesNear the nucleus of animal, fungus, and algalcells is a spherical structure called the centrosome. Prior to the division of a cell, thecentrosome divides into two centrosomes,which travel to opposite ends of the cell


69


during the early phases of cell division. Thecentrosomes contain a pair of structurescalled centrioles, which produce microtubules. These protein tubes form spindlesthat extend toward the nucleus and help thecells chromosomes separate during cell division. Plant cells lack centrioles, but they dohave centrosomes, which serve a functionsimilar to that in animal cells.

CytoskeletonThe cytoskeleton helps the cell maintain itsshape, aids in cellular movement, and helpswith internal movement. Found only ineukaryotic cells, the cytoskeleton is a networkof protein filaments and tubules that extendsthroughout the cytoplasm. Microtubuleshelp form structures such as cilia and flagella,which help in cell movement, and the spindlefibers that help chromosomes move duringcell division. Microfilaments give the cell itsshape and help it contract; intermediate filaments give it strength.

NucleusNear the middle of the cell is the nucleus.The nucleus is the control center of the cell.

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It also contains the structures that transmithereditary traits. A nucleus not undergoingdivision has at least one nucleolus, which isthe site of RNA synthesis and storage.The nucleus is enclosed by a two-layeredmembrane and contains a syrupy nucleoplasm and strands of DNA wrapped aroundproteins in a manner that resembles a stringof beads. Each strand contains a long series ofgenessegments of DNA inherited from theprevious generation. Each gene determinesa heritable characteristic of the organism.Genes also regulate the production of RNA,which in turn controls the manufacture ofspecific proteins.The DNA strands, which are called chromatin because they readily stain with dyes,are usually too thin to be seen with an opticalmicroscope. When a cell begins to divide, thechromatinprotein strands coil repeatedlyaround themselves, condensing into thickerstructures called chromosomes.

How Cells DivideProkaryotes reproduce by several means,including simple fission (in which the celldivides after replicating its DNA) and conjugation (a form of simple sexual reproduction).

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One cell gives rise to two genetically identical daughter cells duringthe process of mitosis. Encyclopdia Britannica, Inc.

Eukaryotic cells, however, undergo a morecomplex division process.The division of eukaryotic somaticcellsthat is, any cell type except germ, orsex, cellsis called mitosis. Between celldivisions, each chromosome makes an exactduplicate of itself and the cells centrosomedivides in two. As mitosis begins, the nucleussignals the chromatin to condense and a body

72


The formation of gametes (sex cells) occurs during the process of meiosis. Encyclopdia Britannica, Inc.

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called the centromere holds each set of original and duplicate chromosomes together.The two centrosomes move to oppositeends of the cell, producing lengths of microtubules called asters and spindle fibers. Thepaired chromosomes become attached toindividual spindle fibers and gather in a lineat mid-cell. The centromeres then split, andthe chromosome pairs are separated; eachmoves along the spindle toward its respective centrosome. Eventually, the cell divides,producing two daughter cellseach with anidentical complement of chromosomes.Each cell of a given species has a characteristic number of chromosomes. Human somaticcells normally contain 46 chromosomes23pairs. Mitosis ensures that both daughter cellshave the full set of chromosomes characteristic of their species.Germ cells produce gametessperm andeggs in humansby meiosis. This involvestwo divisions. During the first division, thechromosomes pair up and duplicate themselves, sometimes exchanging genes througha process called crossing over. The first division produces two cells, each with a full set ofchromosomes. During the second division,the chromosomes in the two cells do notduplicate themselves. The second division

74


produces four gametes, each containing onlyone chromosome from each chromosomepair, or only half the number of chromosomes characteristic of the species. The fullcomplement of chromosomes is restoredwhen a male gamete combines with a femalegamete. For example, a human sperm and ahuman egg each contain 23 chromosomes.When the sperm fertilizes the egg, the twogametes fuse, forming a cell with a complete set of 46 chromosomes. This new cellis called a zygote, and it is the beginning of anew organism.

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C hapter 5History of Cell Theory

T

he history of cell theory is a history of the actual observation ofcells because early prediction andspeculation about the nature of the cell weregenerally unsuccessful. The decisive eventthat allowed the observation of cells wasthe invention of the microscope in the 17thcentury, which stimulated interest in theinvisible world.

Formulation of the TheoryRobert Hooke, who described cork andother plant tissues in 1665, introducedthe term cell because the cellulose wallsof dead cork cells reminded him of theblocks of cells occupied by monks. Evenafter the publication in 1672 of excellentpictures of plant tissues, no significancewas attached to the contents within thecell walls. The magnifying powers of themicroscope and the inadequacy of techniques for preparing cells for observationprecluded a study of the intimate detailsof the cell contents. Beginning in 1673,

76

History of Cell Theory

The cellulose walls of dead cork cells reminded Robert Hooke ofmonks cells. SSPL via Getty Images

Anthony van Leeuwenhoek discoveredblood cells, spermatozoa, and a lively worldof animalcules. A new world of unicellularorganisms was opened up. Such discoveriesextended the known variety of living thingsbut did not bring insight into their basicuniformity. Moreover, when Leeuwenhoekobserved the swarming of his animalcules but failed to observe their division,

77


he could only reinforce the idea that theyarose spontaneously.Cell theory was not formulated for nearly200 years after the introduction of microscopy. Explanations for this delay range fromthe poor quality of the microscopes to thepersistence of ancient ideas concerning thedefinition of a fundamental living unit. Manyobservations of cells were made, but apparently none of the observers could assertforcefully that cells are the units of biologicalstructure and function.Three critical discoveries made duringthe 1830s, when improved microscopes withsuitable lenses, higher powers of magnification, and more satisfactory illuminationbecame available, were decisive events inthe early development of cell theory. First,the nucleus was observed by Scottish botanist Robert Brown in 1833 as a constantcomponent of plant cells. Next, nucleiwere also observed and recognized as suchin some animal cells. Finally, a living substance called protoplasm was recognizedwithin cells, its vitality made evident by itsactive streaming, or flowing, movements,especially in plant cells. After these threediscoveries, cells, previously considered asmere pores in plant tissue, could no longer

78


be thought of as empty, because they contained living material.German physiologist Theodor Schwannand German biologist Matthias Schleidenclearly stated in 1839 that cells are the elementary particles of organisms in bothplants and animals and recognized thatsome organisms are unicellular and others multicellular. Schleiden and Schwannsdescriptive statements concerning thecellular basis of biologic structure arestraightforward andacceptable to modern thought. Theyrecognizedthecommon featuresof cells to be themembrane, nucleus,and cell body anddescribed them incomparisons of various animal and planttissues.

Matthias Schleiden. Kean Collection/Archive Photos/Getty Images

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The Problem of theOrigin of CellsSchwann and Schleiden were not alone incontributing to this great generalization ofnatural science. Strong intimations of the celltheory occur in the work of their predecessors. Recognizing that the basic problem wasthe origin of cells, these early investigatorsinvented a hypothesis of free cell formation,according to which cells developed out of anunformed substance, a cytoblastema, by asequence of events in which first the nucleolus develops, followed by the nucleus, the cellbody, and finally the cell membrane. Eventhough cell division was observed repeatedlyin the following decades, the theory of free cellformation lingered throughout most of the19th century. However, it came to be thoughtof more and more as a possible exception tothe general principle of the reproduction ofcells by division. The correct general principlewas affirmed in 1855 by a German scientist,Rudolph Virchow, who asserted that omniscellula e cellula (all cells come from cells).The inherently complex events of celldivision prevented a quick resolution of thecomplete sequence of changes that occurduring the process. First, it was noted that a

80


Rudolph Virchow. Courtesy of Bildarchiv Preussischer KulturbesitzBPK, Berlin

81


cell with a nucleus divides into two cells, eachhaving a nucleus. Hence, it was concluded thatthe nucleus must divide, and direct divisionof nuclei was correctly described by some.Better techniques created some confusion,because it was found that during cell division the nucleus as such disappears. Also, atthe time of division, dimly observed masses,now recognized as chromosomes, were seento appear temporarily. Observations in the1870s culminated in the highly accuratedescription and interpretation of cell divisionby German anatomist Walther Flemming in1882. His advanced techniques of fixing andstaining cells enabled him to see that cellreproduction involves the transmission ofchromosomes from the parent to daughtercells by the process of mitosis and that thedivision of the cell body is the concludingevent of that reproduction.The discovery that the number of chromosomes remains constant from one generationto the next resulted in the full explanationof the process of meiosis. The description ofmeiosis, combined with the observationthat fertilization is fundamentally the unionof maternal and paternal sets of chromosomes, resulted in the understanding of thephysical basis of reproduction and heredity.

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Rudolph VirchowOne of the most prominent physicians of the19th century, German scientist and statesmanRudolph Virchow pioneered the modern concept of the pathological processes of disease.He emphasized that diseases did not arise inorgans or tissues in general but primarily inindividual cells. Virchow also contributed tothe development of anthropology as a modern science.Rudolph Carl Virchow was born on Oct.13, 1821, in Schivelbein, Prussia. He studied at the University of Berlin and graduatedas a doctor of medicine in 1843. As a youngintern, Virchow published a paper on one ofthe two earliest reported cases of leukemia.His paper became a classic. In 1849 Virchowwas appointed to the chair of pathologicalanatomy at the University of Wrzburgthefirst chair of that subject in Germany. In 1856Virchow became director of the PathologicalInstitute at the University of Berlin.Virchows concept of cellular pathologyreplaced the existing theory that disease arosefrom an imbalance of the four fluid humors ofthe body: blood, phlegm, yellow bile, and blackbile. He applied the cell theory to disease processes and stated that diseased cells arose frompreexisting diseased cells. In 1859 Virchow waselected to the Berlin City Council on which hedealt mainly with such public health matters

83


as sewage disposal, the design of hospitals,meat inspection, and school hygiene. He alsodesigned the new Berlin sewer system. Virchowwas elected to the Prussian National Assemblyin 1861 and to the German Reichstag in 1880.Virchows work in pathological anatomyhad led him to begin anthropological workwith studies of skulls. He was the organizer ofGerman anthropology, and in 1869 he foundedthe Berlin Society for Anthropology, Ethnology,and Prehistory. Virchow died on Sept. 5, 1902,in Berlin, Germany.

Meiosis and fertilization therefore came tobe understood as the complementary eventsin the life cycle of organisms: meiosis halvesthe number of chromosomes in the formationof spores (plants) or gametes (animals), whilefertilization restores the number through theunion of gametes. By the 1890s life in all ofits manifestations could be thought of as anexpression of cells.

84

Conclusion

D

uring the 20th century, biologychanged from a predominantlydescriptive science to one keenlyfounded on experimentation and deductivereasoning. Discoveries such as using antibiotics to treat infectious disease and insulinto treat diabetes, as well as increased knowledge about cell development, were amongthe many important advances made over thepast 100 years or so.In the early 21st century, genetics andmolecular biology have been two of themost active areas of biological research. In2003, exactly 50 years after British biophysicist Francis Crick and American geneticistJames Watson described the double-helicalstructure of DNA, the Human GenomeProject was completed. The 13-year international collaboration of more than 2,800researchersone of the boldest scientificundertakings in historyidentified all ofthe approximately 25,000 human genes anddetermined the sequences of the 3 billionchemical base pairs that make up human

85


DNA. The genetic information providedby the project has enabled researchers topinpoint errors in genes that cause or contribute to disease. In the future, having thetools to know the precise genetic makeup ofindividuals will enable health care providersto deliver truly personalized medicine.

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Glossaryanimalcule A tiny, usually microscopic,organism.archaea Microorganisms of the domainArchaea similar in structure to bacteriathat include methane-producing formsand others of harsh, hot, acidic, or otherwise extreme environments.chromosome A microscopic, threadlikepart of the cell that carries hereditaryinformation in the form of genes.contractile vacuole Membrane-like sacwithin single-celled organisms (likeamoebae and other protozoans) that fillswith water and suddenly contracts, expelling its contents from the cell.glean To gather information or material bitby bit.heritable Able to be inherited or passed onfrom parent to offspring.indefinite Not certain or limited (as inamount or length); having no exact limits.insidious Having a more harmful effectthan is apparent.media A nourishing system for the artificial cultivation of microorganismsor cells.morphology The form and structure of aplant or animal or any of its parts.

87


nucleotide Any of a class of organic compounds, including the structural units ofnucleic acids.oxidize To combine with oxygen; to removehydrogen from, especially by the actionof oxygen.perplexity The state of being puzzled orfilled with uncertainty; bewilderment.phospholipid A type of lipid found in allliving cells in which phosphoric acid anda fatty acid are converted to glycerol.phylogenetic Based on natural evolutionary relationships; acquired in the courseof evolutionary development.protoplasm Usually transparent jellylikesubstance within a cell.protozoa Any of a phylum or subkingdom (Protozoa) of chiefly motile andheterotrophic unicellular protists (asamoebas) that are represented in almostevery kind of habitat and include somedisease-causing parasites of humans anddomestic animals.recombinant Relating to or containinggenetically engineered DNA.solvent A usually liquid substancecapable of dissolving one or more othersubstances.

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Glossary

spermatozoa A motile male sex cell of ananimal usually with rounded or elongatedhead and a long posterior flagellum.spindle Structure made up of protein fibersthat is formed in the cytoplasm duringcell division.synthesis The combination of parts or elements to form a whole.tenet A widely held belief; especially oneheld in common by members of a groupor profession.unprecedented Having no earlier occurrence; original.vacuole A small cavity or space in the tissuesof an organism containing air or fluid.vesicle A small cavity, cyst, or blister usuallyfilled with fluid.

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For More I nformationAmerican Society for Microbiology1752 N StreetWashington, DC 20036-2904(202) 737-3600Web site: http://www.asm.orgThe central organization of microbiologists inthe United States provides information, current microbiology news, and and resourcesfor students interested in microbiology.Bamfield Marine Sciences Centre100 Pachena RdBamfield, BC V0R 1B0Canada(250) 728-3301Web site: http://www.oceanlink.infoA variety of resources are available fromCanadas premier coastal and marinefacility for teaching and research.Center for Bioethics285 Mercer Street, 9th FloorNew York, NY 10003(212) 998-8329Web site: http://bioethics.as.nyu.edu/page/homeA program within New York Universitythat focuses on bioethics education andawareness.

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For More Information

Microbes in Action programUniversity of MissouriOne University BoulevardSt. Louis, MO 63121-4400Web site: http://www.umsl.edu/~microbes/index.htmlThis program is run through University ofMissouris biology program. The goal isto encourage education and appreciationof the role of microorganisms. It providesresources, information, and hands-onactivities relating to microbiology.National Museum of Natural HistoryP.O. Box 37012 Smithsonian Inst.Washington, DC 20013-7012Web site: http://www.mnh.si.eduThe National Museum of Natural History isa part of the Smithsonian Institution, aworld-renowned, state-of-the-art researchcenter and museum. Students can visitthe site or the museum to investigate awide variety of natural history topics.Nature Canada75 Albert Street, Suite 300Ottawa, ON K1P 5E7Canada(800) 267-4088

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Web site: http://www.naturecanada.caNature Canada seeks to protect wildlifeand natural habitats throughout Canada.Outreach programs, publications, andvolunteer support are the primary meansthe organization uses to reach its goals.Virginia Institute of Marine SciencesPO Box 1346Rt. 1208 Greate RoadGloucester Point, VA 23062-1346(804) 684-7000Web site: http://www.vims.eduThe institute, based out of the College ofWilliam and Mary, is both a researchand educational institution on oceansand the study of its systems. It providesinternships, public programs, andadvanced research programs.

Web SitesDue to the changing nature of Internet links,Rosen Educational Services has developed anonline list of Web sites related to the subjectof this book. This site is updated regularly.Please use this link to access the list:http://www.rosenlinks.com/biol/bmtc

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B ibliographyAlberts, Bruce, and others. Essential CellBiology, 3rd ed. (Garland Science, 2009).Betsy, Tom, and Keogh, James Edward.Microbiology Demystified (McGrawHill, 2005).Cohen, Marina. Cells (Crabtree, 2010).Hoagland, Mahlon, and others. Exploringthe Way Life Works: The Science of Biology( Jones & Bartlett, 2001).Kramer, Stephen. Hidden Worlds: LookingThrough a Scientists Microscope (HoughtonMifflin, 2001).Morgan, Sally. Cells and Cell Function(Heinemann Library, 2006).Purves, William, and others. Life: The Scienceof Biology, 7th ed. (W.H. Freeman, 2004).Robinson, Richard, ed. Biology (MacmillanReference, 2002).Stewart, Melissa. Cell Biology (Twenty-FirstCentury Books, 2008).Tocci, Salvatore. Biology Projects for YoungScientists, rev. ed. (Watts, 2000).

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IndexA

problem of origin ofcells, 8084cell wall, 5960centrosomes, 6970cloning, 16, 17Crick, Francis, 85cytoplasm, 5253, 58,60, 61, 63, 64, 66,68, 70cytoskeleton of cell, 70

anatomy, 19

Bbacteriology, 40bioethics, 18biological classification,3334biological warfare, 43biologyabout, 1011areas of study in, 1025history of, 2637modern developments,3637botany, 2930Brown, Robert, 78

DDarwin, Charles, 3435developmental biology,1617DNAdiscovery of structureof, 3637, 85recombinant, 2122, 51

C

E

cell division, 70, 7175,7778, 8082cell membrane, 5659, 79and endocytosis andexocytosis, 5859, 67passive and activetransport, 58cell structure and function, 5275cell theory, history of,7684formulation of theory,7679

ecology, 22embryology, 1617endoplasmic reticulum,6466ethology, 2223eukaryotes, 41, 53, 55, 61,70, 72evolutionary biology,2425evolutionary theory, 15,3435, 36exobiology, 4243

94

Index

F

Linnaean system ofclassification, 1315,3334Linnaeus, Carolus, 13, 33lysosomes, 6869

Fleming, Alexander, 49Flemming, Walther, 82

G

M

genetic engineering, 20,2122genetics, 1920, 3536, 37, 51Golgi complex, 6667, 69Greeks and natural law,2628

meiosis, 7475, 82, 84Mendel, Gregor, 3536microbiology, 3851areas of study, 4043history of, 4751methods of, 4347work of microbiologists, 46microscopes, 4344, 78development of, 31, 32,47, 76electron, 44Middle Ages, study ofbiology in, 28mitochondria, 61,6364, 66mitosis, 7274, 82molecular biology, 1920,37, 51morphology, 19mycology, 41

Hheredity, mechanism of,3536Hooke, Robert, 33, 76Human Genome Project,8586

JJenner, Edward, 47

KKoch, Robert, 4748Kochs Postulate, 48

N

L

natural selection, 3435nucleus of cell, 7071, 78,79, 82

Leeuwenhoek, Anthonyvan, 31, 3233, 47, 7778

95


O

S

organelles, 53, 55, 6170

Schleiden, Matthias, 79, 80Schwann, Theodor, 79, 80sociobiology, 2223stem cell research, 1617

PPasteur, Louis, 4748penicillin, 4950peroxisomes, 69phycology, 41physiology, 19plastids, 6163, 64, 66prokaryotes, 55, 59, 61, 71protozoology, 4041pure cultures, 4547,4849

RRenaissance, study ofbiology during, 2830ribosomes, 66

Ttaxonomy, 1316

Vvaccines, 46, 47vacuoles, 68Virchow, Rudolph, 80, 83virology, 4142

WWallace, Alfred Russel, 34Watson, James, 85

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A Closer Look at Biology, Microbiology, And the Cell