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Chapter 2 The structure of cells（第1页）

Chapter2Thestructureofcells

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&iccellssharethesamebasiclayout，inthattheyaresurroundedbyamembrane，ahwithinwhichthereisavarietyofmembrane-baperformspecializedtasks.ahenucleuswhisDNA.Atroughlyohousandththevolumeoftheeukaryotee，theanizationofaprokaryoteisrelativelystraightforwardbyparison，althoughbacteriapossessverysimilarstructuralaspedmaerytoeukaryoticcells.Theircellmembrahesamelipidbilayer（illdisext）andtheirmolecularmaerysuesforproteinassemblyfunuchthesamewayaseukaryotes.Bacteriadohavespecializedcellstruotilitysuchasflagella，andmaypossessinternalmembrahecaseofagasvacuolethatservesasabuoyancyaid.However，theDNAinabacterialcellisasinglecircularmoledthereisenuent.

Thecellmembrane

Allcellsareenclosedbyaboundarystructure，theplasmamembrane，whichprovidesabarriertootherdtheexternalehoughthemembraaients，unis（bacluded）usuallyhaveextramaterialosideandplantcellsarecharacterizedbyarigidcellwallofcellulose（Figure4）.Overaturyofiionhasshowobeanincrediblypliddynamicmixtureoflipid（fat）moledproteins.Althoughthebasicstructureoftheplasmamembraremelythin-justaoleculesthiingalipidbilayer-itisextremelytoughandflexible，aoallowforthetexoleculesbetweessurroundings.Thisisachievedviathewaterthattlyentersasinaannerbringingsolublemoleculeslikeoxygen（neededasafuel），aissuchasdieexternalmaterialbephysigulfedbythemembrane，aproasphagocytosis.Thereverseprocessisexocytosis，inwhichamembrane-bouerialdestireachesthecellsurface，atwhiembranesfuseahethetswiththerityofthemembrahedynamiatureofthecellmembraneissuchthattheentireplasmamembraurnedover’（replahourlybasis.

&heearliestexperimentsinvolviionsoflipidsaoexplaiiesofmembraneswereperformedtowardstheehtury，oableinGermanybyAgnesPoesstudiedthebehaviourofoilpouredontowaterinaflatdish，identifyingtheinfluenpuritiesonthesurfaoffluids.Sheseh，whowassuffitlyimpressedtogetthempublishediifialNaturein1891.In1932，IrvingLangmuir，winNewYork，rizefthatlipidsspreadonroducealayeronlyohidthatallthemoleculeswereoriehesameway.Thishappensbeeendofthelipidmoleculeisattractedtowater（itishydrophilidtheothereishydrophobic）.Eachlipidmoleculeisshapedlikeanold-fashiohespeg，withthetopofthepegbeingthehydrophilidthetwolegsofthepegrepresentingthehydrion.Allthepegsfloatonthesurfaceofwaterheaddown，legsuppermamonolayer.IerandJamesGreedmembranesfromredblooddingthatmembraneoftwolayers（abilayer）oflipids，makingasandwichwiththehydrophilicpegheadsosides，andthehydrophobitheihereiswaterbothisidethecell，thisarraismaintaihhydrophetherontheihemembrahisarrawasdirectlyvisualizeddecadeslaterwiththeadveronmicroscopy，wherethihighmagnifibrawodarkliedbyalightregiohem（see，forexample，　Figures　4and141935，JamesDanielliandHughDavsohislipidbilayerwasbothsidesbyalayerofproteilasteduntil1972whenSeymarthNisuggestedthattheproteinscouldalsobethreadedthroughthelipidlayers，afromeithersideofthemembrahthisfiguratioraeins，andasiweaveitswaythroughthelipidbilayerseveraltimes.Thelipidmoleamembranearehighlymobile，tlymovingpastohemembraeiothedesofthe‘fluidmosaie.ThisactivityofthelipidswasedbyMichaelEdidinin1970，whehemembranelipidsoftwodiffereypeswitheithergreenorredfluorestchemicals.Thisgavea‘patchwreenlabellingfromindividualcellsgrowherinamixedcellculture.Edidinthenaddedvirusestotheculturewhichcausedthemembranesofadjatcellstofusetoeachother，thusmixingtheirmembranelipidsand，withinanhreenpatchesoffluorescewerereplaelabelling，showiemixingoftheindividuallylabelledlipidmoleculeswithinthefusedmembranes.

&hroughaplantcell，showingthemaindifferenalcells:acellwalloutsidethecellmembrane，chloroplastswithstars（SG）ihem，andalargevathetreofthecell

Withinthisotionofthelipids，groupsofmembraarouhelipidbilayerratherlikeicefloesinthepolarseas.Sometimesafewlipidmoleculeswillformaclusterforafewsedsgspecializedareascalledmembras.Membrasweredislyredtheirfunotyetpletelyuood，althoughitseemslikelytheyareinvolvedinsigweeherearearound500differentmembranelipidswhidandanchordiffereelsthroughthemembraroltheuousflowofmoleculesacrossit.

Theseelsenableaomaintainaninterionofsodiumthatisoheexterion.Thisrequirestpumpingtoremovesodiumwhichotherwisewouldraisetheosmoticpressureinthecellwhiturn，ater，withthepotentialtoburstthecell.Atthesametime，potassiumismaihiamuchhighertratioerhesamemembranepumpbringspotassiumieassodiumispumpedout，anactivitythattakesaboutohirdofthetyofthecell.

Proteihesurfabraasreceptnallingmoleoutsidethecell.Thesemessagesarethenpasseddoweinswithiheoswites，ifrequired，ieredonessusulinwillidirectlywiththemembrane，alloassiuallyeverythingthatgoesoherinfluences，orisinflflflueheabras500typesoflipidmoledupto10，000typesofmembraeins.Cells‘feel’theirimmediatesurroundingswithfififiensionscalledmicrovilli（seeFigure3a–eepithelialcellssuchasthoseresporieheguts，themembranebeesfashiooabrushborder，wheretightlypackedmicrovilliihesurfacearea30-fold（seeFigure14inChapter5）.Whileitisimpossibletoprioritizevariouspartsofthecell，astheyaremutuallyi，withoutmembralifeotexist.

Membranesinsidecells

Membranesarealsocruciallyimportantiworeasons:first，toprovidesurfawhichchemicalreaprodsedlytoprovideseparateareasiheicalreastoproceedwhichmightotherwiseiheachother.Iheinnersurfaceoftheplasmamembrahepositiwithinthedprovidesattatpointsforintracellulartsthatobeinspecifis.Usingtheanalogyofthecellasafactory，theinternalmembranesprovidetheworkbenches，floors，gs，andwallsforallthedifferentpartsofcellprodu，withtherallypositioheoffiwhiformationisstored.Insmallcells，suchasbacteria，whichareusuallyrodshaped，theiheplasmamembraneprovidesalargesurfarelatioerior，sothatanythingthatneedsafixedpositionbe‘hung’ontheinside，alybadotherprenerallyhavelittleornointernalmembraioheinternalvolumeofeukaryotecellsisathousaofabacterium，sothattheeukaryoticcellrequiresavastinternalmembraemaroundahuheareaoftheplasmamembrahisiionofbiochemicalactivitiesiscrucialastherearehundredsofchemicalreasgoingonthatseriouslyiheachother.Prokaryotes，withnointernalmembraestosomeextehisproblembyaggregatinggroupsofspezymesintomultiproteinplexes，whichworkasfreeentitiesiheadditioesediffereabolicprocesseswithinmembraments.Themajorinternalmembraemiiccellsistheendoplasmicreticulum（usuallyshorteoER），whisahroughouttheehispartofthecelliscalledthe（everythihecellmembraneexgthenucleus;Figure5a，b）ahinginitissurrouheplexmixtureofsubstancesdissolvedinwater，likeaveryolecularsoup’.

5.Internalcellmembraructures.（a）Thenuvelope

&esthehe，whisanellesingmito）andendoplasmicreticulum（ER）.（b）Amitosurroundedbyspiralpolyribosomes（r）attachedtothesurfaceoftheER.（uplex，elarrowed.（d）Golgibodies（Go）prisedofstabranes

anelles

Twaheitodriaand，inplants，chloroplasts—havedouble，ratherthansinglemembrahisismostlikelyahangoverfromwhentheywerefree-livingformsearlyincellularevolutioheywereiercell，theirownmembranebecamesurrouhecellmembra.BothmitodriaandchloroplastsA，furtherevideheywere.Therearetwotheoriesastohowchloroplastsandmitodriabecamepartofeukaryoticcells:theycouldhave‘iheeukaryotecellor，alternatively，beenengulfedbyalargeriionshipinwhichbothpart.

&iccellprovideda‘safe’e，inwhichthemitehatcouldbeharvestedbythehostd，inplantcells，chlorlucosebyphotosynthesis.Inmityisprlucosebyaprocesscalledoxidativephosphorylation，whichothesurfaternalmembraae（Figure5b）.Inchloroplasts，glucoseisproducedbyphotosyizymesinstabrahylakoids（Figure4）.

Allothermembrane-banelles（colleownasvacuolesorvesicles）haveasinglebilayermembraypicalcellwillhavearound1000ofthesevadasimilarodria.Secretoryvesitainchemigerssuonesforreleasefromthees，lysosomes，andperoxisomes（seeFigure2）allvariousmixturesofeeinsthatcatalysechemicalreas.Lysosomesbelikeomachofthecell，astheyhydrolytizymesthatbreakdownbiologicalmaterialintoitstpartstoprovidefoodfortheesalsofusewithphagocyticvacuoles（phagotainierialsuchasbacteria，killingaheinvadinganisms.TheBelgiadeDuvediscoveredlysosomes，forwhichhereceivedtheNobelPrizein1974.Healsodiscoveredperoxisomes，whichreplicatebydivisioodriaandchloroplasts，butdoheiroeroxisomesareinvolvediyofbiochemicalpathwaysandatleast50differeheyareimportantinthebreakdown（oxidation）ofsubstancessuchasfats，providingamajorsouretabyina，andplantcells.Beeoftheproductsofoxidatienperoxide，whichisharmfultotheesalsoenzymecalledcatalase，whichbreaksdownthehydrogeer.

Peroxisomesarealsositesofsynthesisofseveralehoseinlivergrespoheproduofbile.Aswithmostindividualaionsinperoxisomeformationhaveseveredaingwillusuallyleadtoafertilizedeggfailiafewdivisions.

&eofproteinproduforthetsofthevariousvacuolessuesandperoxisomesisatthemembraheERwasdiscoveredbythreepioronmicroscopy—KeithPeePaladeinNewYorkandFritiofSjostraheearly1950s.Eleicroscopyallowsa1000-foldiailparedtoallightmicroscopy，butimposesdiffithepreparationofspethataronbeamlypassthroughextremelythiiohediffieioronmicroscopy，Porterandhiscolleaguesopeureinareviouslyunimagiroke，indistindirregularshadowylumpsfromlightmicroscopywerevielydistinellessuchasmiture　5b）.InthewordsofDo，acolleagueofPorterandPalade，‘fists，thede1950to1960heldthesameantiaattendstheopeniiioatlasesofbiologicalultrastruaillclassicstothisday.

&hecell

MitodriawerefirstisolatedbiochemidanalysedbyAlfredLehningerin1949，ingthepreseheenzymesrequiredfeionbyoxidativephosphorylation，ahighlyeffitprowhitsareoxidizedtoproduosiriphosphate（ATP）.Theenergyfwork，buildingproteinsandmovingthingsarouoredinamoleculeofATP.TheePisstoredin‘highenergy’phosphatebonds.Tovolvedbiochemicalprocessshort，thisehereleaseofelethecitricacidcythemitoembraingATPsynthase，aheP.EnergyisreleasedfromATPwheebondsarehydrolysed（aprocesswherethemoleculeissplitintotwopartsbytheadditionofamoleculeofwater）.Withthereleasey，ATPisvertedtoADP（adenosinediphosphate）whituredbacktoATP，staiherelease.

OnlythreeyearsafterLehninger’sbiochemicalcharaitodria，Palade’seleicrographsshmembraure（Figure　5b）.Theorderofthesediscoveriesrefieoveralltrendthatthe‘grindandfind’wsofbiochemistryhaveofteedseminalinformationosiheiractualimagironmicroscope，althoughinthemicroscopist’sview，nothingparewithseeingwhatthetsofthecelllooklike.Thedifferentapproachesofthebiodbiologistbeillustratedasfollows.Presumeherhadeverseenawristwatch，butresehation.Afewdayslaterthebiochemistwouldreportthatthewatchhadbeenanalysedbyseparatingit（grindingitup）intoitspos.Thisanalysiswouldshowthatthewatchwasmadefromvariousproportionsofcopper，brass，steel，ahmaybeafewdiamonds.Thebiologistwouldhatagdohahebadreportthatitseemedtohwhichpoweredaseriescogwheels，thatdrovetwoarmsoofthewatchthatseemedtorotateatatspeed.Whilemassivelyoversimplified，thisparisongivesahediffereheanalyticalapproachofthebiodtheobservationalapproachofthebiologist.Fortunately，usedtogether，thesehavebeenfruitfulindeedincellbiology.

Proteinprodu

&otheER，themajorityofERmembranesarecoveredwithribosomes（Figure　5a）andareknhER，whilsttheremainderbearnoribosomes（smoothER）.Thejobofribosomesistomake（syeinsfromaminoacids，holdingandjoiningtheaminoaakepeptides，theidesaeins.AseriesofRNAmoleculesareieinsynthesis.IhehesequenucleotidebasesfthecodeforaparticularproteinisfirstplateDNAinaprocesscalledtrans，produewmoleessengerRNA（mRNA）.Messeheofthenudergoingmodifi（calledspligtheway.Ooplasm，ribosomesbindtomessengerRNA，whiactsasatemplatefortheliherofaminoatoproteins，aprocesscalledtranslation.

TheaminhttotheribosomebyshortRNAmoleownastransferRNA.ProteiheERehespa）betweentheERmembranes（Figure　5b），wheretheyarefoldedintoafinalfiguratipassedoessuchastheGolgibodies（Figure　5d）.TheGolgibiapparatus）isastackoffiattenedmembranevesicles，whereeinsarepatovacuolesfordistributihoutthedmayalsohavesugarsaddedinaproasgly.hesizedproteiriyd，shouldtheybedefeanyway，theyaretaggedbymoleculesofubiquitinforsroteinmisfoldirimeodisorderssuchascysticfibrosisaeinqualityeismsmaybeelesseffectiveaswegettoAlzheimer’sdiseaseande-relatedives.

&hesizedandfolded，eioreachtheirfiionwithinthegsttheotherbillionsofproteiahesizedanddegraded.Someproteihowomembranebarriersbefthesitewheretheyfulflltheirfun1971，GünterBlobelandDavidSabatinifromtheRockerfellerInstituteinNewYesteda‘signalhypothesis’，inwhisweregivenaluggagelabel，orzipcode，toeheyfiherightdestinatioakestheformofshortsequeninoaoogenials，whiattachtoreceptorproteinstoallowthemthroughmembraoreachthecorreation.In1999，GünterBlobelreceivedtheNobelPrizeforthiswork，laihemolecularmeismsbehindseveraldiseases.Bothcysticfibrosisandprimaryhyperoxaluria（agkidanearlyage）arecausedbyproteinsfailiheircorreatioedthemilliondollarprizemo-warrestruDresdehetryofhisbirth.

Lipidprodu

&sroleihesis，theERisaversatileanellewhibothredtransmitsignalsandactasacellularstoreforcaldisalsorespohesynthesisoflipids.Withinindividualcells，fatisproducedatthesurfaceoftheERastinyindividualdroplets（lipogehoughfatthatwearefamiliarwitharoundtheedgeofoursteaksandoftenaroundourwaistliobeinsolidhomogenouslumps，itallexistswithinmembrane-boundfatdropletsinindividualcellstermedadipocytes（seeFigure3d）.Givenauousirients，adipocyteswillaccumulatemoreandmorelipiddroplets，whichcoalescewiththeirneighbourstobeelargerandlarger，agforthevastmajorityofthee，whireaes‘normal’size.Obesityistlyadisybalaresultsfromtheuedacoflipiddropletswithies.AtthispoiregrettheefficyoftheER，besidesprovidingforfatste，theERalsosynthesizesehesmoothERtobreakdorocesstermedintracellularlipidhydrolysisorlipolysis.Thus，amajorfabodyweightisthebalahesynthesisandbreakdownoflipidsintheER.gthehealthcesofbeiissurprisingthatfatatthecellularlevelhasreceivedrelativelylittleattention，withlipiddropletsthoughtofashaedepots.However，udiesareshowianelles，andanythingbut‘lumpsoffat’.Alleukaryoticcellshavetheabilitytomakelipids，whichproduceallthenaturallyoilsandfats—fromrapeseedandoliveoiliomilkfats，lanolin，andlardinanimalcells.LipidmoleculesaretratedatthesurfaceoftheER，thenpiningadroplet（uniquelysurroundedbyasinglelipidmonolayermembrane）andremaiheER，wheretheecatalyselipidsynthesisarelocated.Mitodriaarecloselyassociatedwiththesitesoflipidprodu，providingtheenergyforfatformatioodriaareactuallytetheredtothesurfaceroupofmembraeins.Asmorelipidisaccumulated，individualdropletsfusewiththeirneighbwhichtheseparatemembraiallyprodugeverlargerdroplets（Figure3d）.Durihisprocessisreversed.Bigdrmeosmallerones，andenzymesfromsmoothERbreakdownthelipidmoleculesthatprhthemembrahesizeofthedropletfromtheoutsidein>

&ionoflipidsalsoocthedintheliningofbloodvessels，particularlyththewallsofmajorarteries.Hereacoflipidsleadstoformationoffattyplaques，resultinginatherosclerofthearteries），whichlimitsbloodflowandthusleadtoheartattadstrokes.Atothersites，iionofbloodflorodueyfailurerene.Excessiveacoflipidsisalsoamajorfatypetwodiabetesaeatosis（fattyliver）.Exptionofalgesitheliverbreaksdownas，leadingtomoreseveressuchascirrhosis.Fortuhefatdropletsstillbebrokendowniheisreversiblewithreduptionofalcohol.Allthisbadhereasoioherwemightteroffwithoutfatcells，buttheyfunrespoionarypressures，allowingfethatmayedussurviveintimese，andalsoallowingmanyothermammalstosurviveseverewintersbyhibernation.