落尘小说

Chapter 1 The nature of cells（第2页）

Mitoi）—arethesiteseionforallcellularauu）—sthe‘blueprint’forallcellactivity，storediheDNA，whiecessitatestaiooplasm。

Nuvelope（hedoublemembraingnud。Theoutermembrainuouswiththeendoplasmicreticulum。

Nuclearpores（NP）—thousandsofelsinthehattroltherapidexgesbetweennud。

Nuo）—siteoftratedRNAandribosomeprodu。

Plasmamembrane（Pm）—alipidbilayerihproteihecell，withspecializedsitesforattaduniwithneighbcells。

Ribosomes（R）—numerous（millionsperaesthatassembleproteins。

Vadvesicles（V）—avarietyofmembraments，whichfulfllspecializedfunparticularcells

speitodria，forexample，produergyforallofthecell’sactivitiesfromthebreakdownoffoodmoleanimalcells。Plantiquelyhavechloroplasts，whivertsunlightandtosugarsasaneoitodria。Bothmitodriaandaythemselveshavebeenfree-livinganismsearlyiion，beforebealyier，plexcell。Everycellhasa‘blueprint’foritsowioheDNAofitsgenes。Inanyparti，theDNAinformatiohesameiype，whethertheyarebrain，gut，orskiatictaintwocopiesofeaAmolecule（theyaresaidtobediploid），exceptthegermlinecells（eggsandsperm）inwhiAexistsasasihesegermlinecellsarehaploid）。ermfusetoproducethefirstbryo（thezygote），twocopiesoftheDored。HowtheDofhaplcellsisreducedwillbedisChapter4。

IheDNAiscirdlieshients，butinplantandaisedwithinananelleknownasthenuLatin　nucula，‘littleisfoldedintoes。Cellswithermedeukaryotes（aGreekword，meaning‘trueker’），rokaryreek，‘beforenuts’）suchasbacteriahaverelativelylittlespecializatioentsiernalanelles。

Allcellsreproducethemselvesbysplittingintotwo。Setoiheirtsrapidlyenoughtodivisionbyaproasbinaryfissioninaslittleas20mihemuchlargereukaryotecellsmay　takethebestpartofadaytodoubletheirsizebeforedivision。Cellsasmaeshaveanunparalleledeffianddiversityofpohebasigblocksofcellareproteinmoledeverycellhastensofthousandsofdifferentproteins，inmillionsofcopies。Aumbersofthevariousmoleaeukaryoticcellareextremelydifficulttoquantify，butestimatesdoexistforbacteria，whichhave40%oftheirvolumemadeupofaroundonemillionmoleculesofsolubleproteins。Avarietyoffivemillionsmallmoleculesatfor3%，thenDNA2%。Thecellmembraerbacterialcell20%，andtheremaihetsaremadeupofthemolecularmaeededtosyeins，ing2000ribosomes。Thesefiguresberoughlyscaledup1000-foldfortheinvolumeofanimalandplantcells，whitainhundredsofindividualanellessuchasmitodria，andaroundtenmillionribosomes。Ribosomesaresmallmolecularmaesthatprovidefortheassemblyofein，acrucialrequiremeeheprovisioeinpriortocelldivision。

Itishardtofindman-mademaerythatmatchesthewsecell。Perhapsthebiggestsuperputerohwouldeclose，butitwouldalsoytophysicallyreproduceitselfwithfasterandfasterprocessors。Whilethismaysouatement，itbeorereasonablewhenyousiderthatcellshavehadaroundfourbilliettheiracttogether，tlydriveingpressuresofion。Ierms，ioifacelladaptstoitse，feedsandreproduitsurvives，butfailuremeahisprocesshasproducedaself-propagatiainingandrepairifunswithalevelofefficyrarelyapproachedbyman-mademaery。Muchoftheeofnanoteology—theengineeringoffunalsystemsatamolecularscale—isaimedatrepliolecularreaswiththeefficylevelsfoundinlivingcells。Aswellastheirhighlyeffitmetabolism（life-sustaiivity），cellsarealsocapableuuralrigidity，asfouhatgivewood，palms，andbamboovaluesofmeiaareexalinas（skystheFarEastoftenuselocalbambooratherthansteelsg）。

Tissuesaion

Themaiicofeukaryoticcellsistheirabilitytoaltertheirshape，pos，aabolismtofulfllaparticulartask—todifferentiate—afacilitywhitoetogetherandformmulticellulartissues，tobiissuesintans，andthenformaireanismsuchasahumanbeing。Weareformedfromaround200differenttypesofcellsthefourmaihelia（surfaectivetissue（blood，bone，andcartilage），muservoustissue。Cellsareproducedatwidelydifferentrates，fromsperm（atarateof1000iimeasaheartbeat）toamaysurvivealifetime。Someofourbloodcellssurvivefhthourswhereastheredcellscirculateforabout120days。

Asanexampleofdifferentiation，webrieflysiderthosecellsthatformourbarriertotheehecellsatthesurfaceofourskinhaveundergoneamajor‘remodelliion）tobeeeocytes，whicharefiattenedpolygonalplatesmademaihesameproteinthatmakesnails，hair，ahersinbirds）。Eaeodsaboutadayatthesurface，befshedaheohatwepresentanewlayerofskiheworldeverydayofourlives。Theshedcellsarereplacedbydivisionofuhatdiffereheypassupwardsthroughthe24celllayersthatmakeuptheoverallthianskihetoplayerofeocyteseffitlyremovesaccumulateddebris，alongwiththe7。5millionbacteriapersquaretimetreandthevariousfungalgrowthsthattlyattempttoizeouroutersurfaces。Thereare1000eocytespersquaremillimetre，andouroverallskinsurfaceareaisjustuwosquaremetres，leadingtoadailyloss（a）ofaroundtwothousandmillioncellseachday。

Theshedskincellsmakeupabout60%ofhouseholddust，andthoseweloseinbedprovidethedailybreadofthemillionorsodustmitesthatliveinourmattresses。Notallpartsofourbodiesarereplacedatthisrate，buttheskin，an，providesagoodexampleofthebasicpropertiesofcells—repli，divisioioasafunalpartofatissue，andultimatelydeath。

Aresingle-ssimple?

Instarktrasttothesinglepurposeiaskincellaspiresto，weshouldsiderwhatlifeislikefle-celledeukaryaheyaresmall，theycouldbesidered‘simple’and‘primitive’。Tosurvive，however，theyfood，mailees，reprodudavoidbeiheranisms。Protozoaarethelargestgroupofsingled-imalserhapsthemostfamiliar。AmoebaehavebeeopursueandcatchotherprotozoasuchasParamecium—perhapstheunicellularequivalentofaliongazebra。Possiblyeveillareagroupofprotozoaoria。Theseunissudroetesparadoxusdoheirfoodbutiachthemselvestovarioussurfadtheirtentadwaitforsomeuozoan，suchasaParamecloseby。Iftheteouched，the‘prey’isinstantlyparalysed，asofitsbodyaresuthetehebodyofthesu，redugthepreytoashrivelledhuskinamatterofmimethodforthetransferofpreytsisunknown，butitisdriveaculararraysoficstructurescalledmicrotubulesioriaacles（see　Figure　6c;microtubuleswillbedesChapter2）。Havinghadagoodmeal，suayate’，aprtheattaodifiedteoeachhwhichtheyexuclei（theyhaveone‘maudthreesmallones）。Ifbingeingaeeshasanfacility。Deeslivesattachedtothegillplatesof　Gammarus，afreshwatershrimp。　Gammarusmularly，sothatDeesrisksbeibehiyshell，losiflowofwaterillplatesahisbrings。However，Deesreizestheearlieststagesofmoulting（possiblybyrespondingtomoultinghormone），aamorphosesintstruownasciliawhichallowsitto‘upstidfiofgillplates（ciliawillbedesChapter2）。Thus，‘simple’single-shavejustasplexalifestyleasthatfoundinmanymultis。

Althoughprotozoa（andsis）mustihtheireosurvive，theirperislargelylimitedtophysidchemiterasattheirmembrawithsfeatures。Insissuchasthegreenalgae　onas，a’isvisiblewithinthechloroplastumicroscope。Theeyespotisaplexsandwibrahrowsofgraaround200differentproteins，ingthesamerhodopsinsfouinaofourowneye。Signalsfromthephotoreceptiveeyespotcausetheflagella（whip-liketailsonthesurfaceofthealgae）tobeatiways，sothealgaeswimthterlight，butawayfromlightthatistht。Whilsttheeyespotmightbethoughtofasarudimehereisnoimaginginvolved，orrequired，astheeyespotsuppliesalltheinformatioheanism’stoight（）rhythms，andoptimizephotosyivity。

Tissueculture

Anenormousamountofinformationaboutmammaliancellshas　invitrostudies;cellsgrowninaglassorplasticflask，maintainedihat37degreesinanatmosphereof5%dioxide，repligthesinthebodyascloselyaspossible。Plantalsobecultured，oftenwiththeircelledoff。Growthofdtissuesoutsidethebodygoesbackasfarasthelate19thtury，ahodologyfortissueaurewasestablishedbyRossHarrisoninBaltimore，USAaround1910。Tissuessugfromlaboratorymiceembryosusuallygrowwellassingleing‘primarycultures’，buttheyalmostalwayshavealimitedlifespan，dyingoffafter50orsoroundsofcelldivisioareisolatedfromsolidtissuesrequireattattothesurfaceoftheplasticgrowthvesseltodivideandgrow，butbloodcells，whiallyexistinaliquid，doadgroension’culture。Cellsareusuallyculturedasasimixedcellculturessuchasthosegrownfromisolatedbonemarrowwillbothsurviveandmaintainthecellulariionsthatothebody。ieceofsolidtissueisexdputintocultures，thecellsthatgrowthemostreadilyarethosethatrespondtowoundinginthelivinganimal。Theseownasfibroblasts，arethecellsthatmaintaiissuethatformsthestructuralframeworkofthebody，suchasligamentsandtendons。Afterinjury，fibroblastspulltheedgesofwouher，ahethatformsscartissue。Iionstheyarelongandthin，withaleadifansoutatthefrontofthecellasitmrowingsurface（Figure　3c）。Withinafewdays，uousroundsofcelldivisiohesurfaceoftheflasktobeeinglycrowded，ahereisnomoreroomfornewlydividedcellstoreattachtsurface，divisiohisissity-depeinhibitionofgrowth，andischaracteristialcells，butnotthosecellsderivedfromtumours。Atthispointthecellsaresub-culturedintonewflasksataredusity。Becauseprimaryculturesdieoffafterabout50divisions，thisputsatimelimitonanyseriesofexperiments，so‘perma’es（usuallyderivedfromtumours）whichdivideielyareoftenpreferredbyresearchers。

&ypesandtheirshape。（a）Haemopoieticell（spherical），（b）epithelial（polygonal）cell，（c）fibroblast，（d）adipocyteorfatcell。Allthesecellshavebeengrowniure。Althoughthesurfaceofthefatcellissmooth，theothercellsaremoretypical，withmembranefolds，ruffles，andfiensions（microvilli）overtheirsurfaces。Thedifferensizebetweedanormalcellisshownbytherouhebottomleft。（e）DeesparadoxusfeedingonaParameciumattachedtoitstentacles

HeLacells，thefirstpermahumane

Althoughpermaesfromhumansaregenerallyderivedfromtumours，tumourbiopsiesarenotoriouslytrickytoestablishihabout1issuccessful。Thefirsttimethathumancellsweregrownincultureasapermaewasin1951，byGeeyinBaltimore，USA。ThesewereHeLacells，socalledbecauseoftheirsource—acervicaltumourbiopsyfromaladyriettaLarietta’swasdiagnosedinJohnsHopkiheveryfewhospitalsintheUSAin1951thatwouldtreatamemberoftheblackpopulationwithouthealthihetime，althoughourbiopsieseretlyputintocultures，nonewerthanafewdays。Hea’scells，hantodoublewithindays，sadlyiheextremelyaggressiveandfastgrowingtumthroughoutherbody，whichkilledherwithinmonths。Thea’scellsveryquicklyspreadaroundtheworldofcellbiology。Iheworldwidedemapermahumane，theTuskegeeInstitutebeganmassprodugthecells，shipping20，000tubesor6trillioncellseveryweek。Thismeansthateveryfewmonths，thesamevolumeofcellsthathadbeeoformHeaherselflefttheprodue。HeLacellswereexperimentedonineveryceivableway，noneofwhichossibleusing‘whole’humans。Theywereexposedtthatmightbetoxiours，everytypeofradiationpossible，aoxinsahemostparttheydidinasignifitlydifferentwaytodifferentspealiaitortanttoshowthathumaurecellsdidnotbehaveinanyuniqueways。Ashercellshavenowbeengroideforoverhalfatury，thereisprobablyenoughofHeheflecells）aroundtheworldforhertopopulateherowntown。Infaehters，afterseeingthefilmJurassicPark，wasbrieflycedthattherewere‘otherinLondon，whereshehadreadthatmuchresearchhadbeeonHeLarietta’swouldprobablyneverhaveknownabouttheirmother’scellsexceptthattheyweretrabyJohnsHopkinsresearcherswhowaoparetheDNAfromHeLacellswithitsanmatch。Thisiedthemtothemultimilliondollarindustrythathadbeeheirmother’stumour，and，notunreasoheirlawyers）triedtoextrathesituation。Uhem，in1951nhtstopatieed，aeofiahassubseque‘aperson’sdiscardedtissuesarenottheirpropertyandercialized’。Evea’sarestilluoaffordhealthiheUSA。Despite60yearsofintensivereseargreAanalysis，theactualreasonsfortheprolifiatureofHeLacellgrowtharestillunknown。HeLaostbesideredthe‘weeds’oftissueculture，havingtakeheresbyatalination，afaetolightwhenculturedcellsbegaerizedbytheirDNAsome2o。

Fordecades，researedonvitrohasbeeioojusthowrepreseis，becauseallyinathree-dimensionaleinamanismatoavarietyofstressesandfaotexperiencedbyasinglelayerofcellsgrolasticdish。Remarkably，inoverhalfaturyoftratedresearch，culturehaveproducedverylittleinthewayofmisleadinginformation，andwithoutthisaccumulatedkisuhetpotentialofstemantherapywouldexistatall。