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Chapter 1 The nature of cells（第1页）

Chapter1Thenatureofcells

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Whatmakesacell?

&hesmallestunitoflife.Everythinglivingisformedofsingle-s，familiartousasbaostplexofcreaturessuchasourselves，formedofmindbogglingnumbersofcells，buttrivialinparisontoumbersintwohuonsofbluewhale.Ihebasigblockoflife，acellmightbesideredarelativelysimplepoiaintainitselfandoallydividieahingcouldbefurtherfromthetruth.Eadeveryplesttothemostplicated，isaself-olecularfactfranticallythroughouteverymislifespahisisthehalfhourofuebacteriabeforetheydivide，ortheselfmaintenandday-to-dayactivityofournervecells，livingforseveraldecades.Theanalogyofacellasafactoryfallssomewhatshortbeatchcellularactivity，thefactoryitselfandmuaerywouldhavetobedismantledaonadailybasis，withoutanyslowingofprodulevels.Bothanimalandplantcellsarearoundathousand-ferthahamuplidiernalanization.

Justwhatsortofchemistrysupporttheextremelevelsofsyallowthesimplercellstodoublethemselvesinminutes，andmoreplicatedcellswithinaday?Atthefuallevel，lifeisbasedoomsofonlysixofthe117ks:，hydren，oxygen，phosphorus，andsulphur.Hydrogenandoxygen，binedasmoleculesof99outofevery100molethecell.Thismightappeartomakelifearatherdiluteaffair，butsomeofthiswateristightlyboundiureermoleddoesnotoccurasactualliquid.Lifeatthemolecularlevelisbasedoedsetofsmalloleonforallcells，whicludesugars（providingchemiergy），fattyaingcellmembranes），aminoacids（theunitsofallproteins），ahesubunitsofinformationalmoleculessuAandDNA）.Allproteinsareformedfromjust20differentaminoacids，whithing.This‘alphabet’ofaminoabinediyofdifferentwayssimilartotheuseofletterstomakewamassive‘vocabulary’ofproteiinaremarkablydiversevarietyoffthestructuralmaterials，chemicalcatalysts，aorsthatsupportaheprocessesoflife.Thecodeforeaiqueproteihercode，thistimeoffourletters，thegenesinourDNAandwhichispassedfrommothercelltodaughtercellateachdivisiohe24，000orsoindividualgenesinourDNAisspecificfleprotein，butourbodiesmayhavemahiseins，producedbymodifyingtheeicmessage.Proteinsarebioformmulti-proteihecogwheelsahatdrivethemotorsofproduaihecell.Thislevelofplexityworksperfeplercellssuchasbacteria，butinlargerandmoreplexcellssuchasourowasksareuakeesitesiermedanelles，aratedfromotherpohiheirownmembranes.Addiherlayerofplexity，ourowain200orsodiffereypes.

Thisbookattemptstoprovideanintroduassivediversityuakenbyoabouttheirbusiness，andwhyany（cellular）shmayresultindisease.

BasiccellcharacteristicsEverythingthatlivesonthesurfaceoftheplaiscellularihispointweshouldexcludeviruses，astheyareuoreproducethemselveswithouthi-jagthesyicprocessesofthecelltheyiheirnon-vitalnatureisemphasizedbythecapaakecrystalsofpurifiedvirusesinsolutiohebasiitoflife，andassuchmustfulfilthreerequirements:（1）tobeaseparateentity，requiringasurfae;（2）toihthesurroundingeraergyinsomewayformaintenandgrowth;and（3）toreplicateitself.Theseparametersarethesameforalllivingbeings，fromthesmallestbacterium，toahe200differeypesthatcreateahumanbeing.Manyanismsliveassinglecells，whereasahumarillionall.Thisnumberparedwiththetotalnumberofpeopleooday（6–8billioalnumberofpeopleestimatedtohaveeverbee（106billion）.Asanaidfortheperoftheseextremelylargenumbers，suseananalogybasedorillionsedsagoequatestoapproximatelythirtythousaimewhehalswereroamingaroundEurope.

Afulywellasasiyor，alternatively，onecellmaybeanindfinitesimallysmallpartofamassiveuwethertomakeasinglebeingsuchasourselves.Inmultis，groupsoftissuesaogethertans.Multicellularityrequirescellslexinternalarchitecture（asweshallseeinChapter2），whereasthesienceofabacteriumallowsforarelativelysimpleanizatioiallyamembranousbaggthenecessarychemiaintainlife）.Whenlifebegan，somefourbillio，thefirstcellswouldhavebeensimilartothebacteriaoftoday.However，simpli’tnecessarilyiiveorunsuccessful，asbacteriaarethemostnumerousandwidespreadofdabraerialfamilycalledArflourishiremeeheplahingelsesurvive.Inoptimalebareproduceevery20miethatwillproduce5billionba11hours，atotheworld’stotalhumanpopulation.Weourselvesareizedbybacteriatothepointwherewehouseteheerial‘guests’

（mai，andweighingaroundonekilogram）thantheualourownbodies.

Membranesandcell>

Asthebasiitoflife，everycellmustbeadistityalyrequiresitsownboundary.Thisboundaryisontoalllifeformsandsistsofathifromtwolayersoffatmolecules（lipids）anddpiercedwithproteinsthatoleculartraffitheditssurroundings.Animalcellsareusuallybioformtissues（forexample，ourskin），invenumbersofdiffereypes.Themembrahesecellsareiact，heldtogetheratspecificattatsites，withothermembraneareasmodifiedtoallowunibetweeissuchasbacteriausuallyhaveara‘cellwall’outsideofthemembraeningadhesivematerialsto‘glue’themtoothercellsortosurfaces（suchasourteeth）.Plantcellshavearigidcellwallwmoleculesofcellulose.Thismajordiffereandaureislargelythereasonwhyanimalsmoveandplants（generally）donot.

Plantcellrovideastrongmeicalframeworkasroteagainstpathogeion.Plantcellwallsareattachedtoeachluemadeofpepolysaccharides（thechemicalthatmakesfruitpreservesset），aheionoflmoleculesofcellulosesandligninsthatarethebasicmaterialsforthetimberaries.Therigidityofthistypeofstruassiveagrowth（e.g.thegiareesofia）evityforthousands.bristlees），butatthesametimerestritstoarootedexistehoughtheirleavesarewellabletoalterpositiontooptimizeexposuretosunlight.

1.Thesizesofatomstorelativesimplewormsonalogseasuredin?ngstroms，a‘dead’unitthatisohofaillindailyusebybiophysicists

&eriorofthecell

Inparisontobacteria，plantandanimalcellsaremassive，aboutohousahevolume.　Figure1showsthescaleoftheunitstheyaremeasuredires（ohofamillimetre）olecularsizesofpos，andwholecellsareusuallytensofmietres（ohousandthofamillimetre）ih.Plantandanimalcellsarealsoinfinitelymoreplitainiructuralelementsbuiltfromproteiypesofinternalmembrane-boundbodiescalledanelles（Figure2）.Individualanelleshave

2.Diagramofthetsofacell

&rosome（CE）—apairoftrioleswhiicrotubulesagtotherequirementsofcellshape，movement，ordivision.

（Cy）—alltentsaresuspendedinaviscousfluidcalledcytosol.

EndoplasmicReticulum（ER）—aworkoffiattenedmembras.Roughendoplasmicreticulum（RER）hasribosomesforproteinsynthesis，smoothER（SER）isinvolvediabolism.

&racellularmatrix（ECM）—materialdepositedoutsidethecellmembrahinlayer，eramounts，suchasorbone.

GA）—aroughlycircularstabraneswhichreceivesfreshlysyeinsfromtheERformodifi，pag，anddistribution.

Lysosomes（L）—vataininglytizymesforbreakdowerialorcelldebris.

Mientsaefilaments（Mf，If）—formthecytoskeleton，inicrotubulestaboutshapedent.