Section { CellDiscovery andTheory The of T:i:{::;|:K@ invention themicros'cope ledto the discovery ofiglls. Section 2 ' ThePlasma Membrane '{i:ﾃｭ;'i::KM Thep|asma membrane helps to maintain a cell's homeostasis. ':. Section 3 andOrganellesiﾂ｡. Structures cells'i iilliii!,i({fﾃ・ukaryotic contain organelles thatallowthe specialization andtheseparation offunctions within thecell. Sectioﾂ｡r 4 Cellular Transport il.il,tlXff!| cellulartransport moves substances within thecell andmoves substances intoand outofthecell.
Abouttentrillioncellsmakeup thehumanbody. Thelargest human cellsareabout thediameter of a humanhair.
The200differenttypesof cells inthehuman bodycome from justonecell.
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Whatis a cell? butonly Allthingsaremadeof atomsandmolecules, orgain livingthingsaretheatomsandmolecules nizedintocells.Inthislab,youwill usea compound microscope to viewslidesof livingthingsand nonliving things.
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with 4[dNB Usethis Foldable thesection, Section7.4.Asyoustudy ineachofthe consider theroleof energy cellular transoort methods discussed.
reviewcontentonl¡newith the Interact¡veTutor,and take Self-Check Qu¡zzes
Structure andFunct¡on Chapter7 . Cellular
Objectives inmicroscope ) Relateadvances
Cell Discoveryand Theory
technology to discoveries about cells. l¡ght ) Comparecompound microscopes withelectron m¡croscopes. thepr¡nciples ofthe ) Summarize celltheory. a ) Differentiate betr¡veen prokaryotic cellanda eukaryotic ce,,.
ffi$ffi@ of cells.
Theinventionof the microscopeled to the discovery
partsofyourbodym¡ghtseem Real-worldReadingLink Thedifferent to pumps your havenothing in common. Yourheart, forexample, bloodthroughout body, whileyoursk¡nprotects andhelps coolyou.Howeve¡ allyourbodyparts haveonethingin common-they arecomposed of cells.
Review Vocabulary organization:theorderly structure ofcells inanorganism
History of the Cell Theory
New Vocabulary cell celltheory ptasma memorane 0rgane e eukaryotic cell nucleus prokaryotic cell
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For centuries,scientistshad no idea that the human body consistsoftrillions ofcells. Cells are so small that their existencewas unknown before the invention ofthe microscope.In 1665,asindicated in Figure 7.1,an English scientistr.ramedRobert Hooke made a simple microscopeand looked at a pieceof cork, the dead cellsofoak bark. Hooke observed small, box-shapedstructures,such asthose shown in Figure 7.2. He called them cellulae(the Latin word meaning small roorrs) becausethe boxlike cells of cork reminded him of the cells in which monks live at a monasterf It is from Hooke'swork that we havethe term c¿ll.A cell is the basicstructural and functional unit ofall living organisms. During the late 1600s,Dutch scientistAnton van Leeuwenhoek (LAY vun hook)-inspired by a book written by Hooke-designed his own microscope.To his surprise, he sawliving organisms in pond water. milk, and various other substances.The work ofthese scientists and others led to new branchesofscience and many new and exciting discoveries.
F¡9ure 7.1
Microscopes in Focus The¡nvention of microscopes, improvements to theinstruments, andnewmicroscope techniqueshaveledto thedevefopment of the / celltheorv and¿ better understandino of cells.
1590 Dutchlensgrinders HansandzachariasJanssen inventthefirstcompound m¡croscope byplac¡ng two lenses in a tube.
182
Chapter7 . Cellular structure andtunction
1565 Robert Hooke observes corkandnames thet¡nychambers thathe seescells.Hepublishes drawings of cells, fleas andotherm¡nute bodies in hisbookMioographia.
1830-f855 scientists discover thecellnucleus (1833)and proposethat both Dlantsand animals are comoosedof cells, fl839).
f683 Dutchbiologisr AntonvanLeeuwenhoek discovers single-celled, animal{ike organisms, nowcalledorotozoans.
The cell theory Naturalistsand scientistscontinuedobservingthe living microscopicworld usingglasslenses.In 1838,Germanscientist MatthiasSchleidencarefullystudiedplant tissuesand concludedthat all plantsarecomposedofcells.A yearlate¡ anotherGermanscientist, TheodorSchwann,reportedthat animal tissuesalsoconsistedofindividual cells.PrussianphysicianRudolphVirchowproposedin 1855that all cellsareproducedfrom the divisionof existingcells.The observations and conclusionsofthesescientistsand othersaresummarizedas the cell theory.The cell theory is one ofthe fundamentalideasofmodern biologyand includesthe followingthreeprinciples: ¡oFigure 7.2 RobertHookeuseda basic l¡ghtm¡croscope to seewhatlookedlike emptychambers in a corksample, lnfet What do you think Hooke would have seen¡f these were living cells?
1. All living organismsarecomposedof one or more cells. 2. Cellsarethe basicunit of structureand organizationof all living orgamsms. 3. Cellsariseonly from previouslyexistingcells,with cellspassing copiesof their geneticmaterialon to their daughtercells.
genetic Réading without checkCancellsappear spontaneously material fromDrevious cells?
Microscope Technology The discovery of cells and the development of the cell theory would not havebeenpossiblewithout microscopes.Improvementsmadeto microscopeshaveenabledscienJísts to study cellsin detail, asdescribedin Figuré 7.1, Turn back to the opening pagesofthis chapterand comparethe magnifications ofthe skin shown there. Note that the detail increasesasthe magnification and resolution-the ability of the microscopeto make individual componentsvisible-increase. Hooke and van Leewenhoek would not havebeen ableto seethe individual structureswithin human skin cellswith their microscopes.Developrnentsin microscopetechnology havegiven scientiststhe ability to study cellsin greaterdetail than early scientistsever thought possible.
f939 Ernest Everett Justwritesthetextbook Bíologyof the Cell Srffaceaftery€arsof studying thestrudure andfunctionof cells.
f880-f890 l-ouis Pasteur andRobert Koch, usingcompound pioneered microscopes, thestudvof bacteria.
[A!'Nm |db ' Review Basedonwhatyou'veread aboutcells, howwouldyounow questions? answer theanalysis
t981 Thescann¡ng tunneling (STM)allows microscope scient¡sts to seeindividual aloms,
Lynn pro. a m¡(robiolog¡st, posesthe ideathat someorganelles found in eukaryotes were oncefree-living pfokafyotes.
In tl¡r¡¡* Interact¡ve TimeLine ú¡¡ncePús ToIearnmoreaboutthese discoveries and other5, v¡sit b¡oloqvqmh.com.g¡ologrf9. . f
Section1-. Cel¡Discoverv andTheorv 183
โ ฌompound light microscopes The modern compound light microscopeconsistsofa seriesofglasslensesand usesvisiblelight to produce a magnified image. Each lens in the seriesmagnifies the image of the previous lens. For example, when two lenseseachindividually magnify 10 times, the total magnificationwould be 100times (10 X 10).Scientrsts often stain cells with dyesto seethem better when using a light microscope becausecellsare so tiny, thin, and translucent.Over the years,scientists have developedvadous techniques and modifications for light microscopes,but the properties ofvisible light will alwayslimit resolution with these microscopes.Objects causelight to scatter,which blurs images.The maximum magnificationwithout blurring is around 1000X.
catrยกiiltts IN tยกIo[o$Y TechnologyRepresentative Companies thatmanufacture scientific equยกpment represenemploy tativesto demonstrate andexplain theยกrproducts to thescientific community. A technology representativeis an expertin thesenew products technology andbringsthis expertise to scientists who mightuse theproducts in the laboratory For moreinformation on biologycareers. visitbยกoloqvqmh.com.
Electron mยกcroscopes As they beganto study cells,scientistsneeded greatermagnification to seethe detailsoftiny parts ofthe cel1.During the secondWorld Waยก in the 1940s,they developedthe electronmicroscope. Insteadoflenses,the electronmicroscopeusesmagnetsto aim a beam of electronsat thin slicesofcells. This type ofelectron microscopeis calleda transmissionelectronmicroscope(TEM) becauseelectronsare passed,or transmitted, through a specimento a fluorescentscreen.Thick parts of the specimenabsorbmore electronsthan thin parts, forming a black-andwhite shadedimage of the specimen.Transmissionelectronmicroscopes can magnify up to 500,000X,but the specimenmust be dead,slicedvery thin, and stainedwith heavymetals. Over the past 65 years,many modifications havebeen made to the original electronmicroscopes.For example,thelcanning electron microscope(SEM) is one modification that directs electronsover the surfaceof the specimen,producing a three-dimensionalimage.One disadvantage ofusing a TEM and an SEM is that only nonliving cellsand tissuescan be observed.To seephotomicrographsmade with electronmicroscopes, visit biofogygmh,comand click on MicroscopyLinks.
Disrover Cells How can you describea new discovety? lmagineyou area scientistlookingthroughthe eyepiece of somenew-fangledinstrumentcalleda microscope and you seea field of similarlyshapedobjects.You might recognizethat the shapesyou seeare not merelycoincidence and randomobjects.Yourwhole ideaol the natureof matteris changingasyou vie\rftheseobjects.
Pยกocedure @glfuE
: L Readand completethe lab safetyform. 2. Preparea datatable in whichyou will recordobservations and drawingsfor threeslides. 3. View the slide imagesyour teacher projectsfor the class.
4. Describe and draw what you see.Besureto ยกncludeenoughdetaยกliri your drawingsto conveythe informationto other scientists who havenot observedcells. Analycls 1. DescribeWhatanalogies or termscouldexplainthe imagesin yourdrawings? 2. Explain How couldyou showHooke,with twenty-first-century technology, that hisfindยกngs werevalid?
t84
Chapter7 . Cellular Structure andFunction
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Another type of microscope,the scanningtunnelingelectronmrcroscope(STM),invoivesbringingthe chargedtip ofa probeextremely closeto the specimensothat the electrons"tunnel" throughthe small gapbetweenthe specimenand the tip. This instrumenthasenabledscicomputerimagesofobjectsassmall entiststo createthree-dimensional asatoms.Unlike TEM and SEM,STM canbe usedwith live specimens. F¡gure7,3 showsDNA, the cellt geneticmaterial,magnifiedwith a scanningtunnelingelectronmicroscope. variousforces The atomicforcemicroscope(AFM) measures betweenthe tip ofa probeand the cell surface.To learn moreabout AFM, readthe CuttingEdgeBiologyfea[)reaf rheend ofthis chapter.
Basic Cell Types Youhavelearned,accordingto the ceiltheor¡ that cellsarethe basrc units ofall living organisms.By observingyour own bodyandthe living things around you, you might infer that ceilsmust exist in variousshapes andsizes.Youalsomight infer that cellsdiffer basedon the functionthey performfor the organism.Ifso, you arecorrect!However,all cellshaveat leastonephysicaltrait in common:theyall havea structurecalleda plasmamembrane.A plasmamembrane,labeledin Figure 7.4, is a spethe cell.Eachof cialboundarythat helpscontrolwhatentersand leaves your skin cellshasa plasmamembrane,asdo the cellsofa rattlesnake. This criticalstructureis describedin detailin the next section. Cellsgenerallyhavea numberof functionsin common.For example,mostcellshavegenéticmaterialin someform that provides instructionsfor makingsubstances that the cell needs.Cellsalsobreak down moleculesto generateenergyfor metaboiism.Scientistshave groupedcellsinto two broadcategories. Thesecategories areprokaryotic (pro kar eeAW tik) cellsand eukaryotic(yewkar eeAW tik) cells. F¡gure7.4showsTEM photomicrographs ofthesetwo celltypes.The imagesofthe prokayoticcell and eukaryoticcell havebeenenlargedso you cancomparethe cell structures.Eukaryoticcellsgenerallyareone to onehundredtimeslareerthan prokaryoticcells.
and of prokaryotic cells Reading Checkcomparethesizes eukaryotic cells. TEM[,lagnilic¿lionr 8000x ColorE¡h¿nc€d
DNA s F¡gure 7,3 Thescann¡ng tunnel¡ng (sTM) provides microscope images, suchasth¡s DNAmolecule ¡nwh¡ch cracks anddepressions andraised areas appear lighter. appear darker Name an applicationfor whichan SfM might be used.
ñ Figuré 7.4 Theprokaryotic cellontheleft issmaller andappears lesscomplex thanthe eukaryot¡c cellonther¡ght. ColoFEnh¿nced TEI\,1 [4agnif kation.8000<
Eukaryot¡ccell andTheory 185 Séct¡on I . CellDiscovery
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Look again at Figure 7.4 and comparethe types ofcells. You can see why scientistsplacethem into two broad categoriesthat are basedon internal structures.Both havea plasmamembrane,but one cell contains many distinct internal structurescalled organelles-specialized structures that carry out specificcell functions. Eukaryotic cells contain a nucleus and other organelles that are bound by membranes,also referred to as membrane-bound organelles.The nucleus is a distinct central organellethat cont¿ins the cel1'sgeneticmaterial in the form of DNA. Organellesenable cell functions to take place in different parts of the cell at the same time. Most organismsare made up of eukaryotic cells and are called eukaryotes.However,some unicellular organisms,such as some algaeand yeast,are also eukaryotes. Prokaryotic cells are defined as cells without a nucleus or other membrane-boundorganelles.Most unicellular organisms,suchasbacteria, areprokaryotic cells.Thus they are calledprokaryotes.Many scientists think that prokaryotesare similar to the first organismson Earth. Origin of cell d¡vers¡ty Ifyou have ever wondered why a company makes two products that are similar, you can imagine that scientists have askedwhy there are two basic types ofcells. The answer might be that eukaryotic cells evolvedfrom prokaryotic cells millions ofyears ago.Accor<iingto the endosymbiont theor¡ a symbiotic mutual relationship involved one prokaryotic cell living inside of another. The endosymbiont theory is discussedin greaterdetail in Chapter 14. Imagine how organismswould be different ifthe eukaryotic form had not evolved.Becauseeukaryotic cells are larger and have distrnct organelles,thesecells havedevelopedspecificfunctions. Having specific functions has led to cell diversit¡ and thus more diverseorganisms that can adapt better to their environments. Life-forms more complex than bacteria might not haveevolvedwithout eukaryotic cells.
Ssstisr¡ Z¿jlt Sedion Summary havebeenusedasa ) lVicroscopes toolforscientific study since the late1500s. usedifferent typesof ) Scientists microscopes to studycells. summarizes three ) Thecelltheory principles. areh¡vobroadgroups of ) There celltypes-prokaryotic cellsand eukaryotic cells.
Understand Main Ideas 1. ffiffi@ Explainhowthe develooment andimDrovement of microscopes changed thestudyof living organisms. 2. Compare andcontrasta com' pound lightmicroscope andan electron mrcroscope. 3. Summarize thecelltheory ¿. Differentiate theolasma membrane andtheorqanelles.
cells conta¡n a nucleus ) Eukaryotic andorganelles.
Chapter7 . Cellular Structure andFunction
5. De¡onbahowyouwoulddetermineifthecells ofa newly discoveredorganism wereprokaryotic or eukaryotic. 6. GmBiology lf theoverallmagnification of a series of h¡ro lenses is30X,andonelensmagnrfied5x, whatisthemagnification of theotherlens? Calculate thetotal magnification if the5X lensis replaced bya 7x iens. \
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Objectives ) Describehowa cell'splasma membrane functions. ldentify.the rolesof proteinl ) in andcholesterol carbohydrates, thepยกasma membrane.
Review Vocabulary ion: an atomor groupof atomswith electriccharge a positiveor negatยกve
The PlasmaMembrane Theplasmamembranehelpsto maยกntaina cell's W@ homeostasis, youmightpass yourschool, Real-World Readingl-ink Whenyouapproach grounds. Thisfenceprevents theschool thatsunounds through a gatein a fence people studentt andthegateallows notbetherefromentering whoshould parents have a structure cells and eukaryotic cells to enter. Prokaryotic staff,and of theirinternal envยกronments. thatmaintaยกns control
New Vocabulary permeability selective phospholipid bilayer transportprotern model fluidmosaic
3"Figure 7,5 captures fishwhile Left:Thefishnetselectively allowingwaterandotherdebristo pass through. selects theplasmamembrane Right:similarly, substances entering andleavingthe cell.
Function of the PlasmaMembrane Recall from Chapter 1 that the processof maintaining balance in an organism'sinternal environment is called homeostasis.Homeostasisis essentialto the survival of a cell. One ofthe structures that is primarily responsiblefor homeostasisis the plasma membrane. The plasma membrane is a thin, flexible boundary between a cell and its environment that allows nutrients into the cell and allows waste and other products to leavethe cell. All prokaryotic cells and eukaryotic cells have a plasma membrane to separatethem from the watery environments in which they exist. A key property of the plasma membrane is selective permeability (pur mee uh BIH luh tee), by which a membrane allows some substancesto passthrough while keeping others out. Consider a fish net as an analogy ofselective permeability. The net shown in Figure 7.5 has holes that allow water and other substancesin the water to pass through but not the fish. Depending on the size ofthe holes in the net, some kinds of fish might passthrough, while others are caught. The diagram in Figure 7.5 illustrates selectivepermeability ofthe plasma membrane. The arrows show that substancesenter and leavethe cell through the plasma membrane. Control of how, when, and how much ofthese substancesenter and leavea cell relies on the structure ofthe plasma membrane.
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Polar heads Cholesterol lnsidethe cell Transport protein ¡. Figure 7,6 lhe phospholipid bilayerlooks likea sandwich, with thepolarheadsfacingthe outs¡deandthe nonpolar tailsfacingthe inside. lnÍef How do hydrophobic substances cfoss a plasma membrane?
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Polar Science usage: having an unequal distribution of charge. Thepositiveend of a polnr molecule ah:ractsthe negatiyeend of a polar molecule. Common usage: telating to a geographic pole or region. Thepolar ícecap in Greenlandís,on a v e r a gLeo, k m t h i c k . . . , . , . . . . , . . . . . .
188
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Structure of the PlasmaMembrane (![!!!fts/;r Most of the moleculesin the plasma membrane are lipids. Recall from Chapter 6 that lipids are large molecules that are composedof glycerol and three fatty acids. Ifa phosphate group replacesa fatty acid, a phospholipid forms. A phospholipid (fahs foh LIH pid) is a molecule that has a glycerol backbone,two fatty acid chains, and a phosphate-containinggroup. The plasma membrane is composed of a phospholipid bilayer, in which two layers of phospholipids are arranged tail-to-tail, as shown in Figure 7,6. In the plasma membrane, phospholipids arrange themselvesin a way that allows the plasma membrane to exist in the watery environment. The phospholipid bilayer Notice in Figure 7.6 that eachphospholipid is diagrammed as a head with two tails. The phosphategroup in eachphospholipid makes the head polar. The polar head is attracted to water becausewater also is polar. The two fatty acid tails are nonpolar and are repelled by water. The two layersof phospholipid molecules make a sandwich, with the fatty acid tails forming the interior of the plasma membrane and the phospholipid headsfacing the watery environments found inside and outside the cell, as shown in Figure 7,6, This bilayer structure rs critical for the formation and function of the plasma membrane. The phospholipids are arranged in such a way thaithe polar headscan be closestto the water moleculesand the nonpolar tails can be farthest away from the water molecules. When many phospholipid molecules come together in this manner, a barrier is createdthat is polar at its surfacesand nonpolar in the middle. Water-solublesubstanceswill not move easily through the plasma membrane becausethey are stopped by the nonpolar middle. Therefore, the plasma membrane can separatethe environment inside the cell from the environment outside the cell.
Chapter 7 . Cellular Structure andFunction
Other components of the plasma membran€ Moving with and among the phospholipids in the plasma membrane are cholesterol, proteins, and carbohydrates' When found on the outer surface of the plasma membrane, proteins called receptors transmit sienals to the inside of the cell' Proteins at the inner surface anchor tlie plasma membrane to the cell's internal support structure, giving the cell its shape.Other proteins span the entire membrane and create tunnels through which certain substancesenter and leavethe cell' These transport proteins move needed substancesor waste materials through the plasma membrane, and therefore contribute to the selective permeability of the plasma membrane. (f
Question Session workwith
andaskeachotherquestions a Dartner Discuss membrane. abouttheplasma Askasmany eachother'sanswers. questions asyouthinkof whiletaking turns.
neading check Describe the benefitof a bilayerstructurefor the plasmamemDrane
Locatethe cholesterolmoleculesin Figure 7.6' Nonpolar cholesterol is repelledby water and is positioned among the phospholipids.Cholesterol helps to prevent the fatty-acid tails ofthe phospholipid bilayer from sticking together,which contributes to the fluidity of the plasmamembrane. Although avoiding a high-cholesteroldiet is recommended, cholesterolplays a critical role in plasmamembrane structure and it is an important substancefor maintaining homeostasisin a cell. Other substancesin the membrane, such as carbohydratesattatched proteins, stick out from the plasma membrane to define the cell's to characteristicsand help cells identify chemical signals.For example, carbohydratesin the membrane might he$ disease-fightingcells recognize and attack a potentialiy harmful cell.
&ünw.s tlaxJtfiHAr,Wttr Easedon Rea¡Data* $.*ggs";+w'v lri;r¡i\,q.¡¡s+*isi!$ $:b*l $1$ lnvolvedin the death Howareproteinchannels of nervecellsafter a stroke?A strokeoccurs
Deúa and Obs€ntaffons
when a bloodclot blocksthe flow of oxygencontainingbloodin a portionof the brain.Nerve cellsin the brainthat releaseglutamatearesensitiveto the lackof oxygenand releasea flood of glutamatewhen oxygenis low.Duringthe glutaThis mateflood,the calciumpumpisdestroyed. affectsthe movementof calciumionsinto and calout of nervecells.Whencellscontainexcess isdisrupted. cium,homeostasis Th¡nk Cr¡t¡cálly 1. Interpret Howdoesthe glutamateflood destroythe calciumpump? levelswere 2. Predictwhat would happenif Ca'z. stroke. during a loweredin the nervecell *Dataobt¿ined destroyed. D.W2005.Neurodegener¡tion:cellulardefen(es from:Choi, ^/¿ture 433:696598.
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Togethe¡ the phospholipids in the bilayer createa "sea" in which other moleculescan float, like applesfloating in a barrel ofwater. This "sea" concept is the basisfor the fluid mosaic model of the plasma membrane. The phospholipids can move sidewayswithin the membrane just as applesmove around in water. At the same time, other components in the membrane, such as proteins, also move among the phospholipids. Becausethere are different substancesin the plasma membrane, a pattern, or mosaic, is createdon the surface.You can see this pattern in Figure 7.7. The components of the plasma membrane are in constant motion, sliding past one another.
Sectiq&t¿** Section Summary
Understand Main ldeas permeability isa properly of 1. WW@ ) Selective Describehowthe theplasma membrane thatallows plasma ¡tto membrane helps mainta¡n (ontrolwhatenters andleaves theceu. homeostasis ina cell. 2. Explainhowtheinside membrane ismade ofa upof ) Theplasma cell remains twolayers of phospholipid separate from its molecules. environment. proteins andtransport ) Cholesterol 3. Diagramtheplasma membrane; a¡dinthefunction oftheplasma label each component. membrane. 4. ldentifythemolecules inthe model describes the I Thefluidmosaic plasma provide membrane that plasrna membrane. basic membrane structure, cell identity, andmembrane fluidity.
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Chapter7 . Cellular Structure andFunct¡on
Exo{'a,irt whal elfectmorecholesterólin theplasma membrane will haveon the membrane.
tMElIEIEbniobsy Using whatyouknowaboutthe term/ros¿lr,writea paragraph describing another biological mosaic,
Objectives and ldentยกfy thestructure t 'functยกon of thepartsof a typical cell. eukaryotic ) Compareand contrast cells. structures of plantandanimal Review Vocabulary upthe thatspeeds enzyme: a protein reaction rateof a chemicรกl New Vocabulary cytoplasm cytos keleton ftbos0me nucte0tus endoplasmic reticulum Golgiapparatus vacuole lysosome centrยกole mitochondrion chloroplast cellwall cilium flagellum
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Structuresand Organelles that allow the cellscontainorganelles Eukaryotic Wffi@ of functionswithin the cell. and the separatยกon specialization
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youstarta company to manufacture hiking a ReadingLink Suppose Real-tlllorld butit wouldbe e pairo{bootscouldbemadeindividually byoneperson, Each boots. GA eukaryotic cellshavespecialized line" Similarly, moreefficยกent to useanassembly specific tasks,muchlikea factory structures thatperform t) 1รญ
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You just haveinvestigatedthe part ofa cell that functions asthe boundary betweenthe inside and outsideenvironments.The environment inside the plasma membrane is a semifluid material called cytoplasm. In a prokaryotic cell, all ofthe chemicalprocessesofthe cell, such asbreaking down sugarto generatethe energyusedfor other functions, take placedirectly in the cltoplasm. Eukaryotic cellsperform theseprocesseswithin organelles in their cytoplasm.At one time, scientiststhought that cell organelles floated in a seaof cytoplasm. More recentlยก cell biologists have discovered that organelles do not float freely in a cell,but are supportedby a structure within the cยกoplasm similar to the structure shown in Figure 7,8. The cytoskeleton is a supporting network oflong, thin protein fibers that form a framework for the cell and provide an anchor for the organelles inside the cells. The cytoskeleton also has a function in cell movement and other cellular activities. The cytoskeletonis made ofsubstructures called microtubules and microfilaments. Microtubules are long, hollow protein cylindersthat form a rigid skeletonfor the cell and assistin moving substanceswithin the cell. Microfilaments are thin protein threadsthat help give the cell shapeand enablethe entire cell or parts of the cell to move. Microtubules and microfilaments rapidly assembleand disassembleand slide past one another. This allows cellsand orsanellesto move.
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Figure 7.9 Compare theillustrations ofa plant cel, animal cell, andprokaryotic celi. Some organelles areonlyfouncl inplant cells-others, onlyin animal cells. Prokaryotic cells donothave membrane bound orqanelles. Nucleus
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Chapter7 . Céllular Structure andFunct¡on
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Cell Structures In a factor¡ there are seParateareasset up for performing different tasks. Eukaryotic cells also have separateareasfor tasks. Membranebound organelles make it possible for different chemical processesto take place at the same time in different parts ofthe cytoplasm' such as protein syntheOrganellescarry out essentialcell processes, food, excretion ofwastes, and of digestion transformation, sis,-energy and functjon' You structure unique has a cell division. Each organelle lines, and assembly can compare organelles to a factory's offices' other important areasthat keep the factory running. As you read about the different organelles, refer to the diagrams ofplant and animal cells in F¡gure 7.9 to seethe organelles of each type.
SSo¡i¡:e¡mcl¡¡'i Cytoplasm Cytoskeleton ryfe- prefix; from Greek,meaning ce\|.............'
The nucleus ]ust as a factory needsa manager,a cell needsan organelle to direct the cell processes.The nucleus,shown in Figure 7'10, is the cellt managing structure. It contains most of the cell'sDNA, which storesinformation used to make proteins for cell growth, function, and reoroduction. The nucleus is surrounded by a double membrane called the nuclear envelope.The nuclear envelopeis similar to the plasma membrane, exceptlhe nuclear membrane has nuclear pores that allow larger-sized subsiancesto move in and out ofthe nucleus.Chromatin, which is a complex DNA attached to protein, is spreadthroughout the nucleus'
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theroleofthenucleus. neadingcheckDescribe
Ribosomes One of the functions of a cell is to produceproteins' The organellesthat help manufacture proteins are called ribosomes. Ribosomesare made of two components-RNA and protein-and are not js bound by a membrane like other organelles.Within the nucleus the site of ribosome production called the nucleolus' shown in F¡gure 7'1o' Cells havé many ribosomes that produce a variety ofproteins that are usedby the cell or are moved out and usedby other cells' Someribosomesfloat freely in the cytoplasm, while others are bound to another organelle called the endoplasmic reticulum. Free-floating ribosomes produce proteins for use within the cytoplasm ofthe cel1.Bound ribo,omes p-du." proteins that will be bound within membranesor used by other cells.
'q Figure 7.f 0 Thenucleus of a cellis a Thephotomicroshape. three-dimensional of a nucleus graphshowsa cross-section lnlet Explainwhy all the cross-sedionsof a nucleusarc not ident¡cal.
Nucleafpore 560x Magnification: ColorEnhancedTE[¡
Nuclear envelope
andorganelles193 section3 . Structures
Color-Enhanced TEll
Rough endoplasmic reticulum
Ribosome
Smooth endoplasmic reticulum '-rFigure 7.lf Ribosomes aresimple structures madeof RNAandproteinthatmay be attached to thesurface of the rough endoplasm¡c reticulum. Theylooklikebumps ontheendoplasm¡c reticulum.
Endoplasmic ret¡culum The endoplasmic reticulum (en duh PLAZ mihk . rih TIHK yuh lum), also called ER, is a membrane systemoffolded sacsand interconnectedchannelsthat servesas the site for protein and lipid synthesis.The pleatsand folds ofthe ER provide a large amount of surfaceareawhere cellular functions can take place.The area of ER where ribosomes are attachedis called roush endoplasmicreticulum. Notice in Figure 7,tt that the rough ER appearsto havebumps on it. Thesebumps are the attachedribosomes that will produce proteins for export to other cells. F¡gure 7.11also showsthat there are areasofthe ER that do not have ribosomesattached.The areaof ER where no ribosomesare attachedis called smooth endoplasmicreticulum. Although the smooth ER has no ribosomes,it doesperform important functions for the cell. For example, the smooth ER provides a membrane surfacewhere a variety of complex carbohydratesand lipids. including phospholipids,are synthesized.Smooth ER in the liver detoxifiesharmful substances.
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Data and Observations How is vesicletraffic from the ERto the Golgi apparatusregulated? someproteinsaresynthesizedby ribosomes on the endoplasmic reticulum (ER)Theproteinsareprocessed in the ER,and vesicles containing theseproteinspinchoff and m¡grateto the Golgiapparatus. sc¡entists currently arestudy¡ng the molecules that areinvolvedin fusingthesevesicles to the Golgiapparatus. Th¡nk Cr¡t¡Gally l. Interpret a Diagram Nametwo complexes on the Golg¡apparatus that mightbe involved. in vesiclefusion. 2, Hypothesizean explanationfor vesicletransport basedon what you havereadaboutcvtoplasmand the cytoskeleton.
"Dataobtalned from:Brittle, E.E.,andWatert[4.G.2000. ER"to-golgi traffic-thisbud'sforyo!. Sc/ence289:403-404.
t94
Chapter7 . Cellular Structure andFunction
leavingthe Golgiapparatus Ves¡cle
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Golg¡ apparatus
¡"rF¡gure 7.f2 Flattened stacksof membranes makeuptheGolgiapparatus.
After the hiking boots are made in the factor¡ Golgi apparatus they must be organized into pairs, boxed, and shipped. Similarl¡ after proteins are made in the endoplasmic reticulum, some might be transferred to the Golgi (GAWL jee) apparatus,illustrated in Figure 7.12. The Golgi apparatus is a flattened stack of membranesthat modifies, sorts, and packagesproteins into sacscalled vesicles.Vesiclesthen can fuse with the cell's plasma membrane to releaseproteins to the environment outside the cell. Observethe vesiclein Figure 7'12. Vacuoles A factory needsa place to store materials and wasteproducts. Similarly, ce1lshave membrane-bound vesiclescalled vacuolesfor temporary storageof materials within the cytoplasm. A vacuole, such as the plant vacuole shown in F¡gure 7'13, is a sacused to store food, enzymes,and other materials neededby a cell. Somevacuolesstore waste products. Interestingly, animal cells usually do not contain Yacuoles.If animal cells do havevacuoles,thev are much smaller than those in nlant cells. vacuole
$ F¡gure 7.'13 Plantcellshavelarge storagecompartments membrane-bound calledvacuoles.
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andorganelles Section 3 . Structures
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\o F¡gure 7,14 Lysosomes containdigestive enzymes thatcanbreakdownthewastes contained in vacuoles.
Lysosome
Lysosomes Factoriesand cells also need clean-up crews.In the cell, lysosomes, shown in F¡gure 7.14, are vesiclesthat contain substances that digest excessor worn-out organellesand food particles. Lysosomes also digest bacteria and viruses that haveenteredthe cell. The membrane surrounding a lysosomepreventsthe digestiveenzymes inside from destroying the cell. Lysosomescan fuse with vacuolesand dispensetheir enzymes into the vacuole,digesting the wastesinside. Centrioles Previously in this section you read about microtubules and the cytoskeleton.Groups of microtubules form another strucrure called a centriole (SEN tree ol). Centrioles, shown in F¡gure 2.15, are organellesmade of microtubules that function durinq cell division. Centrioles are located in the cytoplasm of animal celi-sand most protists and usually are near the nucleus.You will learn about cell division and the role ofcentrioles in Chaoter 9.
,' Figure 7.15 Centrioles aremade of microtubules andplaya roleincelldivision
t96
Chapter7 . Cellular Strucfure andFunct¡on
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Mitochondrion
lnner membrane
0uter memDrane
Imagine now that the boot factory has its own genMitochondria produces the electricity it needs.Cells also have energy erator that called mitochondria (mi tuh KAHN dree uh; singular, generzLtors mitochondrion) that convert fuel particles (mainly sugars)'into usable energy.Figure 7.16 shows that a mitochondrion has an outer membrane and a highly folded inner membrane that provides a large surface area for breaking the bonds in sugar molecules.The energy produced from that breakageis stored in the bonds ofother moleculesand later used by the cell. For this reason,mitochondria often are referred to as the "powerhouses"of cells. Factory machines need electricity that is generatedby ChloroP¡asts burning fossil fuels or by collecting energy from alternative sources, such as the Sun. Plant cells have their own way ofusing solar energy. In addition to mitochondria, plants and some other eukaryotic cells contain chloroplasts' which are organellesthat capture light energy and convert it to chemical energy through a processcalled photosynthesis. Examine Figure 7'17 and notice that inside the inner membrane are many smal1,disk-shaped compartments cailed thylakoids. It is here that the energy from sunlight is trapped by a pigment called chlorophyll. Chlorophyll gives leavesand stems their green color' Chloroplasts belong to a group ofplant organellescalled plastids, some of which are used for storage.Some plastids store starchesor lipids. Others, such as chromoplasts, contain red, orange, or yellow pigments that trap light cnergy and give color to plant structures such asflowers or leaves.
¡,' Figure 7.16 Mitochondria makeenergy available to thecell. Descr¡be t te membÍanestructureof a mitochondrion.
'! Figure 7.17 In plantt chloroplasts captureandconvertlighfenergyto chem¡cal energy.
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Ihylakoid
Chloroplast andorganelles197 Section 3 . Structures
Planlcell1 s Figure 7.f8 The¡llustration showsplant cellsandtheircellwalls. Compare thisto the transmission electronm¡crograph showinglhe cellwallsof adjacent plantcells.
e F¡gure 7.19 Thehairlikestructures in rhe photomicrograph arecilia,andthetaiflike structures areflagella.Bothstructures function in cellmovement. lnler Wherein the body of an animat would you predict cilia might be found?
Cifia on the surface of a para mecium
t9a Chapter7 . Cellular Structure andFunction
Cell wall Another structureassociated with plant cellsis the cell wall, shownin Figure 7.18,The cell wall is a thick, rigid, meshof fibers.thatsurroundsthe gutsideofthe plasmamembiane,protecting the cell and giving it support.Rigid cellwallsallow plantstó standat variousheights-from bladesofgrassto Californiaiedwoods.plant cellwalls aremadeofa carbohydrate calledcellulose,which sivesthe wallitsinflexible characterisrics. Table7.1listscellwalls anávarious othercell structures. Cilia and flagella Someeukaryoticcell surfaceshavestructures calledcilia and flagellathat projectoutsidethe plasmamembrane.As shownin Figure 7.19,cilia (singular,cilium) aie short,numerouspro_ jectionsthat look like hairs.The motion of cilia is similar to the motion ofoars in a rowboat.Flagella(singular,flagellum)arelongerdnd less numerousthan 4lia. Theseprojectionsmovewith a whiplike motion. Cilia and flagellaarecomposedof microtubulesarransedin a 9 + 2 configuration,in which nine pairsof microtubulessuriound two sinele microtubules.Typicall¡ a cell hasoneor two flasella. - Prokaryoticcilia and flagellacontain cytoplásmand are enclosed by_theplasmamembrane.They consistofproiein building blocks. While both structuresareusedfor cell movement,cilia ariealsofound on stationarycel1s.
Bacter¡a with flagella
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more TableToexplore Int€ractive visit aboutcellstructures, biologyqmh.com.
Summaryof Cel!Structures
Plantcells, fungicells, andsomeprokaryotes
Cellwall
the supportandprotects banierthat provides An inflexible plantcell
Centrioles
forcell thatoccurin pairsandareimportant Organelles division
Plantcellsonly
Chloroplast
containing withthylakoids organelle A double-membrane takesplace wherephotosynthesis chlorophyll
Cilia
and thataidin locomotion fromcellsurfaces Proiections alongsurfaces feeding;alsousedto sweepsubstances
Some animalcells, protistcelltand prokaryotes
for the cellwithinthecytoplasm A framework
cells All eukaryotic
thatisthes¡teof protein A highlyfoldedmembrane synthesis
cells Alleukaryotic
andfeeding thataidin locomotion Projections
Someanimalcells. prokaryotes, and someplantcells cells All eukaryotic
Golgiapparatus
that modifies A flattenedstackof tubularmembranes outside thecell themfordistribution proteins andpackages
Lysosome
for thebreakdown. Animalcellsonly enzymes digestive thatcontains A vesicle substances cellular or worn-out of excess
M¡tochondrion
cells available Alleukaryotic energy thatmakes organelle A membrane-bound to the restof the cell cells Alleukaryotic
Nucleus
for codeddirections controlcenterof thecellthatcontains andcelldivision of proteins theproduction
cells Alleukaryotic
membrane Plasma
of themovement thatcontrols boundary A flexible intoandoutof thecell substances synthesis thatisthesiteof protein 0rqanelle
Allcells
storage of forthetemporary vesicle A membrane-bound materials
large; Plantcells-one few animalcells-a small
Animalcellsand . mostprotistcells
Cytoskeleton
Endoplasmlc
reticulum
Flagella
Ribosome
andorganellesl9!) Section3. Structures
Comparing Cells Table 7.'l summarizes the structures ofeukaryotic plant cells and animal cells.Notice that plant cells contain chlorophyll-they can capture and transform energy from the Sun into a usableform of chemical energy.This is one ofthe main characteristicsthat distinguishesplants from animals. In addition, remember that animal cells usually do not contain vacuoles.Ifthey do, vacuolesin animal cells are much smaller than vacuolesin plant cells.Also, animal cells do not have cell walls. Cell walls give plant cells protection and support.
{raltfinR$ IN ¡HOt d¡üV ScienceCommun¡cations Specialist Manypublishers of sc¡entific material hirecommun¡cations spec¡al¡sts to writeaboutresearch and itsimportance to thegeneral public. thisoftenis accomplished through pressreleaselad1pamphleb, and targeted mailings. Formoreinformationon biology careertvisit biologygmh.com.
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Organellesat Work With a basic understanding of the structures found within a cell, it becomeseasierto envision how those structures work together to perform cell functions. Take, for example,the synthesisofproteins. Protein synthesisbegins in the nucleus with the information contained in the DNA. Genetic information is copied and transferred to another genetic molecule called RNA. Then RNA and ribosomes, which havebeen manufactured in the nucleolus, leavethe nucleus through the pores ofthe nuclear membrane. Together,RNA and ribosomes man-ufacfrre proteins. Eachproteinmadeon the rough ERhasa particularfunction;it mighr become a protein that forms a part of the plasma membrane, a protein that is releasedfrom the cell or a protein transportedto other organilles.Other ribosomes will float freely in the cytoplasm and make proteins aswell. Most ofthe proteins made on the surfaceofthe ER are sent to the Golgi apparatus.The Golgi apparatuspackagesthe proteins in vesicles and transports them to other organellesor out ofthe cell. Other organellesusethe proteins to carry out cell processes.For example,lysosomes useproteins, enzymesin particula¡ to digestfood and waste.Mitochon_ dria use enzymesto produce a usableform ofenergy for the cell. After reading about the organellesin a cell, it becomesclearerwhy peopleequatethe cell to a factor¡ Each organellehas its job to do, and the health ofthe cell dependson all ofthe componentsworking together.
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Section Summary
Understand Main Ideas
cellscontainmembrane. ) Eukaryot¡c boundorganelles in thecytoplasm that performcellfunct¡ons.
1 . W@ ldentify the roleof thenucleus in a eukaryotic cell.
arethe sitesof protein ) Ribosomes synthes¡s. arethe powerhouses ) Mitochondria of cells. ) Plantandanimalcellscontain many of thesameorganelles, whileother organelles areunique to eitherplant cellsor animalcells.
Chapter7 . Cellular Structure andFunctton
Summarizetheroleof theendoplasmic reticulum. Analogy Makea flowchart comparing the partsof a cellto an automobile production line. Infer whysomescientists do not consider ribosomes to becell 0rganel¡es.
5. l*ypothosizo howlysosomes wouldbeinvolved inchanging a caterpillar intoa butterfly.
6. @ElEilE@$iotogy Categorize thestructures andorganellesin Table7.1¡ntolistsbased on celltype,thendrawa concept map illustrating yourorganization.