Chapter 1 The nature of cells（第1页）

Chapter1Thenatureofcells

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。

Microtubules（Mt）—dynamiccytoskeletalpos，whistantlyassembledaiditywithioplasm，andactas‘rails’foriransport。