Cell membrane - Wikipedia

The cell membrane is a biological membrane that separates the interior of all cells from the outside environment (the extracellular space) which protects ... Cellmembrane FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Biologicalmembranethatseparatestheinteriorofacellfromitsoutsideenvironment IllustrationofaEukaryoticcellmembrane ComparisonofEukaryotesvs.Prokaryotes Thecellmembrane(alsoknownastheplasmamembrane(PM)orcytoplasmicmembrane,andhistoricallyreferredtoastheplasmalemma)isabiologicalmembranethatseparatestheinteriorofallcellsfromtheoutsideenvironment(theextracellularspace)whichprotectsthecellfromitsenvironment.[1][2]Thecellmembraneconsistsofalipidbilayer,includingcholesterols(alipidcomponent)thatsitbetweenphospholipidstomaintaintheirfluidityatvarioustemperatures.Themembranealsocontainsmembraneproteins,includingintegralproteinsthatgoacrossthemembraneservingasmembranetransporters,andperipheralproteinsthatlooselyattachtotheouter(peripheral)sideofthecellmembrane,actingasenzymesshapingthecell.[3]Thecellmembranecontrolsthemovementofsubstancesinandoutofcellsandorganelles.Inthisway,itisselectivelypermeabletoionsandorganicmolecules.[4]Inaddition,cellmembranesareinvolvedinavarietyofcellularprocessessuchascelladhesion,ionconductivityandcellsignallingandserveastheattachmentsurfaceforseveralextracellularstructures,includingthecellwall,thecarbohydratelayercalledtheglycocalyx,andtheintracellularnetworkofproteinfiberscalledthecytoskeleton.Inthefieldofsyntheticbiology,cellmembranescanbeartificiallyreassembled.[5][6][7][8] Contents 1History 2Composition 2.1Lipids 2.2Phospholipidsforminglipidvesicles 2.3Carbohydrates 2.4Proteins 3Function 4Prokaryotes 5Structures 5.1Fluidmosaicmodel 5.2Lipidbilayer 5.3Membranepolarity 5.4Membranestructures 5.5Cytoskeleton 5.6Intracellularmembranes 5.7Variations 6Permeability 7Seealso 8Notesandreferences 9Externallinks History Mainarticle:HistoryofcellmembranetheoryWhileRobertHooke’sdiscoveryofcellsin1665ledtotheproposaloftheCellTheory,Hookemisledthecellmembranetheorythatallcellscontainedahardcellwallsinceonlyplantcellscouldbeobservedatthetime.[9]Microscopistsfocusedonthecellwallforwellover150yearsuntiladvancesinmicroscopyweremade.Intheearly19thcentury,cellswererecognizedasbeingseparateentities,unconnected,andboundbyindividualcellwallsafteritwasfoundthatplantcellscouldbeseparated.Thistheoryextendedtoincludeanimalcellstosuggestauniversalmechanismforcellprotectionanddevelopment.Bythesecondhalfofthe19thcentury,microscopywasstillnotadvancedenoughtomakeadistinctionbetweencellmembranesandcellwalls.However,somemicroscopistscorrectlyidentifiedatthistimethatwhileinvisible,itcouldbeinferredthatcellmembranesexistedinanimalcellsduetointracellularmovementofcomponentsinternallybutnotexternallyandthatmembraneswerenottheequivalentofacellwalltoplantcell.Itwasalsoinferredthatcellmembraneswerenotvitalcomponentstoallcells.Manyrefutedtheexistenceofacellmembranestilltowardstheendofthe19thcentury.In1890,anupdatetotheCellTheorystatedthatcellmembranesexisted,butweremerelysecondarystructures.Itwasnotuntillaterstudieswithosmosisandpermeabilitythatcellmembranesgainedmorerecognition.[9]In1895,ErnestOvertonproposedthatcellmembranesweremadeoflipids.[10] Thelipidbilayerhypothesis,proposedin1925byGorterandGrendel,[11]createdspeculationtothedescriptionofthecellmembranebilayerstructurebasedoncrystallographicstudiesandsoapbubbleobservations.Inanattempttoacceptorrejectthehypothesis,researchersmeasuredmembranethickness.[9]In1925itwasdeterminedbyFrickethatthethicknessoferythrocyteandyeastcellmembranesrangedbetween3.3and4 nm,athicknesscompatiblewithalipidmonolayer.Thechoiceofthedielectricconstantusedinthesestudieswascalledintoquestionbutfuturetestscouldnotdisprovetheresultsoftheinitialexperiment.Independently,theleptoscopewasinventedinordertomeasureverythinmembranesbycomparingtheintensityoflightreflectedfromasampletotheintensityofamembranestandardofknownthickness.TheinstrumentcouldresolvethicknessesthatdependedonpHmeasurementsandthepresenceofmembraneproteinsthatrangedfrom8.6to23.2 nm,withthelowermeasurementssupportingthelipidbilayerhypothesis.Laterinthe1930s,themembranestructuremodeldevelopedingeneralagreementtobethepaucimolecularmodelofDavsonandDanielli(1935).Thismodelwasbasedonstudiesofsurfacetensionbetweenoilsandechinodermeggs.Sincethesurfacetensionvaluesappearedtobemuchlowerthanwouldbeexpectedforanoil–waterinterface,itwasassumedthatsomesubstancewasresponsibleforloweringtheinterfacialtensionsinthesurfaceofcells.Itwassuggestedthatalipidbilayerwasinbetweentwothinproteinlayers.Thepaucimolecularmodelimmediatelybecamepopularanditdominatedcellmembranestudiesforthefollowing30years,untilitbecamerivaledbythefluidmosaicmodelofSingerandNicolson(1972).[12][9] Despitethenumerousmodelsofthecellmembraneproposedpriortothefluidmosaicmodel,itremainstheprimaryarchetypeforthecellmembranelongafteritsinceptioninthe1970s.[9]Althoughthefluidmosaicmodelhasbeenmodernizedtodetailcontemporarydiscoveries,thebasicshaveremainedconstant:themembraneisalipidbilayercomposedofhydrophilicexteriorheadsandahydrophobicinteriorwhereproteinscaninteractwithhydrophilicheadsthroughpolarinteractions,butproteinsthatspanthebilayerfullyorpartiallyhavehydrophobicaminoacidsthatinteractwiththenon-polarlipidinterior.Thefluidmosaicmodelnotonlyprovidedanaccuraterepresentationofmembranemechanics,itenhancedthestudyofhydrophobicforces,whichwouldlaterdevelopintoanessentialdescriptivelimitationtodescribebiologicalmacromolecules.[9] Formanycenturies,thescientistsciteddisagreedwiththesignificanceofthestructuretheywereseeingasthecellmembrane.Foralmosttwocenturies,themembraneswereseenbutmostlydisregardedthisasanimportantstructurewithcellularfunction.Itwasnotuntilthe20thcenturythatthesignificanceofthecellmembraneasitwasacknowledged.Finally,twoscientistsGorterandGrendel(1925)madethediscoverythatthemembraneis“lipid-based”.Fromthis,theyfurtheredtheideathatthisstructurewouldhavetobeinaformationthatmimickedlayers.Oncestudiedfurther,itwasfoundbycomparingthesumofthecellsurfacesandthesurfacesofthelipids,a2:1ratiowasestimated;thus,providingthefirstbasisofthebilayerstructureknowntoday.Thisdiscoveryinitiatedmanynewstudiesthatarosegloballywithinvariousfieldsofscientificstudies,confirmingthatthestructureandfunctionsofthecellmembranearewidelyaccepted.[9] Thestructurehasbeenvariouslyreferredtobydifferentwritersastheectoplast(deVries,1885),[13]Plasmahaut(plasmaskin,Pfeffer,1877,1891),[14]Hautschicht(skinlayer,Pfeffer,1886;usedwithadifferentmeaningbyHofmeister,1867),plasmaticmembrane(Pfeffer,1900),[15]plasmamembrane,cytoplasmicmembrane,cellenvelopeandcellmembrane.[16][17]Someauthorswhodidnotbelievethattherewasafunctionalpermeableboundaryatthesurfaceofthecellpreferredtousethetermplasmalemma(coinedbyMast,1924)fortheexternalregionofthecell.[18][19][20] Composition Cellmembranescontainavarietyofbiologicalmolecules,notablylipidsandproteins.Compositionisnotset,butconstantlychangingforfluidityandchangesintheenvironment,evenfluctuatingduringdifferentstagesofcelldevelopment.Specifically,theamountofcholesterolinhumanprimaryneuroncellmembranechanges,andthischangeincompositionaffectsfluiditythroughoutdevelopmentstages.[21] Materialisincorporatedintothemembrane,ordeletedfromit,byavarietyofmechanisms: Fusionofintracellularvesicleswiththemembrane(exocytosis)notonlyexcretesthecontentsofthevesiclebutalsoincorporatesthevesiclemembrane'scomponentsintothecellmembrane.Themembranemayformblebsaroundextracellularmaterialthatpinchofftobecomevesicles(endocytosis). Ifamembraneiscontinuouswithatubularstructuremadeofmembranematerial,thenmaterialfromthetubecanbedrawnintothemembranecontinuously. Althoughtheconcentrationofmembranecomponentsintheaqueousphaseislow(stablemembranecomponentshavelowsolubilityinwater),thereisanexchangeofmoleculesbetweenthelipidandaqueousphases. Lipids Examplesofthemajormembranephospholipidsandglycolipids:phosphatidylcholine(PtdCho),phosphatidylethanolamine(PtdEtn),phosphatidylinositol(PtdIns),phosphatidylserine(PtdSer).Thecellmembraneconsistsofthreeclassesofamphipathiclipids:phospholipids,glycolipids,andsterols.Theamountofeachdependsuponthetypeofcell,butinthemajorityofcasesphospholipidsarethemostabundant,oftencontributingforover50%ofalllipidsinplasmamembranes.[22][23]Glycolipidsonlyaccountforaminuteamountofabout2%andsterolsmakeuptherest.InRBCstudies,30%oftheplasmamembraneislipid.However,forthemajorityofeukaryoticcells,thecompositionofplasmamembranesisabouthalflipidsandhalfproteinsbyweight. Thefattychainsinphospholipidsandglycolipidsusuallycontainanevennumberofcarbonatoms,typicallybetween16and20.The16-and18-carbonfattyacidsarethemostcommon.Fattyacidsmaybesaturatedorunsaturated,withtheconfigurationofthedoublebondsnearlyalways"cis".Thelengthandthedegreeofunsaturationoffattyacidchainshaveaprofoundeffectonmembranefluidityasunsaturatedlipidscreateakink,preventingthefattyacidsfrompackingtogetherastightly,thusdecreasingthemeltingtemperature(increasingthefluidity)ofthemembrane.[22][23]Theabilityofsomeorganismstoregulatethefluidityoftheircellmembranesbyalteringlipidcompositioniscalledhomeoviscousadaptation. Theentiremembraneisheldtogethervianon-covalentinteractionofhydrophobictails,howeverthestructureisquitefluidandnotfixedrigidlyinplace.Underphysiologicalconditionsphospholipidmoleculesinthecellmembraneareintheliquidcrystallinestate.Itmeansthelipidmoleculesarefreetodiffuseandexhibitrapidlateraldiffusionalongthelayerinwhichtheyarepresent.[22]However,theexchangeofphospholipidmoleculesbetweenintracellularandextracellularleafletsofthebilayerisaveryslowprocess.Lipidraftsandcaveolaeareexamplesofcholesterol-enrichedmicrodomainsinthecellmembrane.[23]Also,afractionofthelipidindirectcontactwithintegralmembraneproteins,whichistightlyboundtotheproteinsurfaceiscalledannularlipidshell;itbehavesasapartofproteincomplex. Inanimalcellscholesterolisnormallyfounddispersedinvaryingdegreesthroughoutcellmembranes,intheirregularspacesbetweenthehydrophobictailsofthemembranelipids,whereitconfersastiffeningandstrengtheningeffectonthemembrane.[4]Additionally,theamountofcholesterolinbiologicalmembranesvariesbetweenorganisms,celltypes,andeveninindividualcells.Cholesterol,amajorcomponentofanimalplasmamembranes,regulatesthefluidityoftheoverallmembrane,meaningthatcholesterolcontrolstheamountofmovementofthevariouscellmembranecomponentsbasedonitsconcentrations.[4]Inhightemperatures,cholesterolinhibitsthemovementofphospholipidfattyacidchains,causingareducedpermeabilitytosmallmoleculesandreducedmembranefluidity.Theoppositeistruefortheroleofcholesterolincoolertemperatures.Cholesterolproduction,andthusconcentration,isup-regulated(increased)inresponsetocoldtemperature.Atcoldtemperatures,cholesterolinterfereswithfattyacidchaininteractions.Actingasantifreeze,cholesterolmaintainsthefluidityofthemembrane.Cholesterolismoreabundantincold-weatheranimalsthanwarm-weatheranimals.Inplants,whichlackcholesterol,relatedcompoundscalledsterolsperformthesamefunctionascholesterol.[4] Phospholipidsforminglipidvesicles Lipidvesiclesorliposomesareapproximatelysphericalpocketsthatareenclosedbyalipidbilayer.[24]Thesestructuresareusedinlaboratoriestostudytheeffectsofchemicalsincellsbydeliveringthesechemicalsdirectlytothecell,aswellasgettingmoreinsightintocellmembranepermeability.Lipidvesiclesandliposomesareformedbyfirstsuspendingalipidinanaqueoussolutionthenagitatingthemixturethroughsonication,resultinginavesicle.Bymeasuringtherateofeffluxfromthatoftheinsideofthevesicletotheambientsolution,allowsresearchertobetterunderstandmembranepermeability.Vesiclescanbeformedwithmoleculesandionsinsidethevesiclebyformingthevesiclewiththedesiredmoleculeorionpresentinthesolution.Proteinscanalsobeembeddedintothemembranethroughsolubilizingthedesiredproteinsinthepresenceofdetergentsandattachingthemtothephospholipidsinwhichtheliposomeisformed.Theseprovideresearcherswithatooltoexaminevariousmembraneproteinfunctions. Carbohydrates Plasmamembranesalsocontaincarbohydrates,predominantlyglycoproteins,butwithsomeglycolipids(cerebrosidesandgangliosides).Carbohydratesareimportantintheroleofcell-cellrecognitionineukaryotes;theyarelocatedonthesurfaceofthecellwheretheyrecognizehostcellsandshareinformation,virusesthatbindtocellsusingthesereceptorscauseaninfection[25]Forthemostpart,noglycosylationoccursonmembraneswithinthecell;rathergenerallyglycosylationoccursontheextracellularsurfaceoftheplasmamembrane.Theglycocalyxisanimportantfeatureinallcells,especiallyepitheliawithmicrovilli.Recentdatasuggesttheglycocalyxparticipatesincelladhesion,lymphocytehoming,[25]andmanyothers.Thepenultimatesugarisgalactoseandtheterminalsugarissialicacid,asthesugarbackboneismodifiedintheGolgiapparatus.Sialicacidcarriesanegativecharge,providinganexternalbarriertochargedparticles. Proteins Type Description Examples Integralproteinsortransmembraneproteins Spanthemembraneandhaveahydrophiliccytosolicdomain,whichinteractswithinternalmolecules,ahydrophobicmembrane-spanningdomainthatanchorsitwithinthecellmembrane,andahydrophilicextracellulardomainthatinteractswithexternalmolecules.Thehydrophobicdomainconsistsofone,multiple,oracombinationofα-helicesandβsheetproteinmotifs. Ionchannels,protonpumps,Gprotein-coupledreceptor Lipidanchoredproteins Covalentlyboundtosingleormultiplelipidmolecules;hydrophobicallyinsertintothecellmembraneandanchortheprotein.Theproteinitselfisnotincontactwiththemembrane. Gproteins Peripheralproteins Attachedtointegralmembraneproteins,orassociatedwithperipheralregionsofthelipidbilayer.Theseproteinstendtohaveonlytemporaryinteractionswithbiologicalmembranes,andoncereacted,themoleculedissociatestocarryonitsworkinthecytoplasm. Someenzymes,somehormones Thecellmembranehaslargecontentofproteins,typicallyaround50%ofmembranevolume[26]Theseproteinsareimportantforthecellbecausetheyareresponsibleforvariousbiologicalactivities.Approximatelyathirdofthegenesinyeastcodespecificallyforthem,andthisnumberisevenhigherinmulticellularorganisms.[24]Membraneproteinsconsistofthreemaintypes:integralproteins,peripheralproteins,andlipid-anchoredproteins.[4] Asshownintheadjacenttable,integralproteinsareamphipathictransmembraneproteins.Examplesofintegralproteinsincludeionchannels,protonpumps,andg-proteincoupledreceptors.Ionchannelsallowinorganicionssuchassodium,potassium,calcium,orchlorinetodiffusedowntheirelectrochemicalgradientacrossthelipidbilayerthroughhydrophilicporesacrossthemembrane.Theelectricalbehaviorofcells(i.e.nervecells)arecontrolledbyionchannels.[4]Protonpumpsareproteinpumpsthatareembeddedinthelipidbilayerthatallowprotonstotravelthroughthemembranebytransferringfromoneaminoacidsidechaintoanother.ProcessessuchaselectrontransportandgeneratingATPuseprotonpumps.[4]AG-proteincoupledreceptorisasinglepolypeptidechainthatcrossesthelipidbilayerseventimesrespondingtosignalmolecules(i.e.hormonesandneurotransmitters).G-proteincoupledreceptorsareusedinprocessessuchascelltocellsignaling,theregulationoftheproductionofcAMP,andtheregulationofionchannels.[4] Thecellmembrane,beingexposedtotheoutsideenvironment,isanimportantsiteofcell–cellcommunication.Assuch,alargevarietyofproteinreceptorsandidentificationproteins,suchasantigens,arepresentonthesurfaceofthemembrane.Functionsofmembraneproteinscanalsoincludecell–cellcontact,surfacerecognition,cytoskeletoncontact,signaling,enzymaticactivity,ortransportingsubstancesacrossthemembrane. Mostmembraneproteinsmustbeinsertedinsomewayintothemembrane.[27]Forthistooccur,anN-terminus"signalsequence"ofaminoacidsdirectsproteinstotheendoplasmicreticulum,whichinsertstheproteinsintoalipidbilayer.Onceinserted,theproteinsarethentransportedtotheirfinaldestinationinvesicles,wherethevesiclefuseswiththetargetmembrane. Function Adetaileddiagramofthecellmembrane Illustrationdepictingcellulardiffusion Thecellmembranesurroundsthecytoplasmoflivingcells,physicallyseparatingtheintracellularcomponentsfromtheextracellularenvironment.Thecellmembranealsoplaysaroleinanchoringthecytoskeletontoprovideshapetothecell,andinattachingtotheextracellularmatrixandothercellstoholdthemtogethertoformtissues.Fungi,bacteria,mostarchaea,andplantsalsohaveacellwall,whichprovidesamechanicalsupporttothecellandprecludesthepassageoflargermolecules. Thecellmembraneisselectivelypermeableandabletoregulatewhatentersandexitsthecell,thusfacilitatingthetransportofmaterialsneededforsurvival.Themovementofsubstancesacrossthemembranecanbeeither"passive",occurringwithouttheinputofcellularenergy,or"active",requiringthecelltoexpendenergyintransportingit.Themembranealsomaintainsthecellpotential.Thecellmembranethusworksasaselectivefilterthatallowsonlycertainthingstocomeinsideorgooutsidethecell.Thecellemploysanumberoftransportmechanismsthatinvolvebiologicalmembranes: 1.Passiveosmosisanddiffusion:Somesubstances(smallmolecules,ions)suchascarbondioxide(CO2)andoxygen(O2),canmoveacrosstheplasmamembranebydiffusion,whichisapassivetransportprocess.Becausethemembraneactsasabarrierforcertainmoleculesandions,theycanoccurindifferentconcentrationsonthetwosidesofthemembrane.Diffusionoccurswhensmallmoleculesandionsmovefreelyfromhighconcentrationtolowconcentrationinordertoequilibratethemembrane.Itisconsideredapassivetransportprocessbecauseitdoesnotrequireenergyandispropelledbytheconcentrationgradientcreatedbyeachsideofthemembrane.[28]Suchaconcentrationgradientacrossasemipermeablemembranesetsupanosmoticflowforthewater.Osmosis,inbiologicalsystemsinvolvesasolvent,movingthroughasemipermeablemembranesimilarlytopassivediffusionasthesolventstillmoveswiththeconcentrationgradientandrequiresnoenergy.Whilewateristhemostcommonsolventincell,itcanalsobeotherliquidsaswellassupercriticalliquidsandgases.[29] 2.Transmembraneproteinchannelsandtransporters:Transmembraneproteinsextendthroughthelipidbilayerofthemembranes;theyfunctiononbothsidesofthemembranetotransportmoleculesacrossit.[30]Nutrients,suchassugarsoraminoacids,mustenterthecell,andcertainproductsofmetabolismmustleavethecell.Suchmoleculescandiffusepassivelythroughproteinchannelssuchasaquaporinsinfacilitateddiffusionorarepumpedacrossthemembranebytransmembranetransporters.Proteinchannelproteins,alsocalledpermeases,areusuallyquitespecific,andtheyonlyrecognizeandtransportalimitedvarietyofchemicalsubstances,oftenlimitedtoasinglesubstance.Anotherexampleofatransmembraneproteinisacell-surfacereceptor,whichallowcellsignalingmoleculestocommunicatebetweencells.[30] 3.Endocytosis:Endocytosisistheprocessinwhichcellsabsorbmoleculesbyengulfingthem.Theplasmamembranecreatesasmalldeformationinward,calledaninvagination,inwhichthesubstancetobetransportediscaptured.Thisinvaginationiscausedbyproteinsontheoutsideonthecellmembrane,actingasreceptorsandclusteringintodepressionsthateventuallypromoteaccumulationofmoreproteinsandlipidsonthecytosolicsideofthemembrane.[31]Thedeformationthenpinchesofffromthemembraneontheinsideofthecell,creatingavesiclecontainingthecapturedsubstance.Endocytosisisapathwayforinternalizingsolidparticles("celleating"orphagocytosis),smallmoleculesandions("celldrinking"orpinocytosis),andmacromolecules.Endocytosisrequiresenergyandisthusaformofactivetransport. 4.Exocytosis:Justasmaterialcanbebroughtintothecellbyinvaginationandformationofavesicle,themembraneofavesiclecanbefusedwiththeplasmamembrane,extrudingitscontentstothesurroundingmedium.Thisistheprocessofexocytosis.Exocytosisoccursinvariouscellstoremoveundigestedresiduesofsubstancesbroughtinbyendocytosis,tosecretesubstancessuchashormonesandenzymes,andtotransportasubstancecompletelyacrossacellularbarrier.Intheprocessofexocytosis,theundigestedwaste-containingfoodvacuoleorthesecretoryvesiclebuddedfromGolgiapparatus,isfirstmovedbycytoskeletonfromtheinteriorofthecelltothesurface.Thevesiclemembranecomesincontactwiththeplasmamembrane.Thelipidmoleculesofthetwobilayersrearrangethemselvesandthetwomembranesare,thus,fused.Apassageisformedinthefusedmembraneandthevesiclesdischargesitscontentsoutsidethecell. Prokaryotes Prokaryotesaredividedintotwodifferentgroups,ArchaeaandBacteria,withbacteriadividingfurtherintogram-positiveandgram-negative.Gram-negativebacteriahavebothaplasmamembraneandanoutermembraneseparatedbyperiplasm,however,otherprokaryoteshaveonlyaplasmamembrane.Thesetwomembranesdifferinmanyaspects.Theoutermembraneofthegram-negativebacteriadifferfromotherprokaryotesduetophospholipidsformingtheexteriorofthebilayer,andlipoproteinsandphospholipidsformingtheinterior.[32]Theoutermembranetypicallyhasaporousqualityduetoitspresenceofmembraneproteins,suchasgram-negativeporins,whicharepore-formingproteins.Theinner,plasmamembraneisalsogenerallysymmetricwhereastheoutermembraneisasymmetricbecauseofproteinssuchastheaforementioned.Also,fortheprokaryoticmembranes,therearemultiplethingsthatcanaffectthefluidity.Oneofthemajorfactorsthatcanaffectthefluidityisfattyacidcomposition.Forexample,whenthebacteriaStaphylococcusaureuswasgrownin37◦Cfor24h,themembraneexhibitedamorefluidstateinsteadofagel-likestate.Thissupportstheconceptthatinhighertemperatures,themembraneismorefluidthanincoldertemperatures.Whenthemembraneisbecomingmorefluidandneedstobecomemorestabilized,itwillmakelongerfattyacidchainsorsaturatedfattyacidchainsinordertohelpstabilizethemembrane.[33]Bacteriaarealsosurroundedbyacellwallcomposedofpeptidoglycan(aminoacidsandsugars).Someeukaryoticcellsalsohavecellwalls,butnonethataremadeofpeptidoglycan.Theoutermembraneofgramnegativebacteriaisrichinlipopolysaccharides,whicharecombinedpoly-oroligosaccharideandcarbohydratelipidregionsthatstimulatethecell'snaturalimmunity.[34]Theoutermembranecanbleboutintoperiplasmicprotrusionsunderstressconditionsoruponvirulencerequirementswhileencounteringahosttargetcell,andthussuchblebsmayworkasvirulenceorganelles.[35]Bacterialcellsprovidenumerousexamplesofthediversewaysinwhichprokaryoticcellmembranesareadaptedwithstructuresthatsuittheorganism'sniche.Forexample,proteinsonthesurfaceofcertainbacterialcellsaidintheirglidingmotion.[36]Manygram-negativebacteriahavecellmembraneswhichcontainATP-drivenproteinexportingsystems.[36] Structures Fluidmosaicmodel AccordingtothefluidmosaicmodelofS.J.SingerandG.L.Nicolson(1972),whichreplacedtheearliermodelofDavsonandDanielli,biologicalmembranescanbeconsideredasatwo-dimensionalliquidinwhichlipidandproteinmoleculesdiffusemoreorlesseasily.[37]Althoughthelipidbilayersthatformthebasisofthemembranesdoindeedformtwo-dimensionalliquidsbythemselves,theplasmamembranealsocontainsalargequantityofproteins,whichprovidemorestructure.Examplesofsuchstructuresareprotein-proteincomplexes,picketsandfencesformedbytheactin-basedcytoskeleton,andpotentiallylipidrafts. Lipidbilayer Diagramofthearrangementofamphipathiclipidmoleculestoformalipidbilayer.Theyellowpolarheadgroupsseparatethegreyhydrophobictailsfromtheaqueouscytosolicandextracellularenvironments. Lipidbilayersformthroughtheprocessofself-assembly.Thecellmembraneconsistsprimarilyofathinlayerofamphipathicphospholipidsthatspontaneouslyarrangesothatthehydrophobic"tail"regionsareisolatedfromthesurroundingwaterwhilethehydrophilic"head"regionsinteractwiththeintracellular(cytosolic)andextracellularfacesoftheresultingbilayer.Thisformsacontinuous,sphericallipidbilayer.Hydrophobicinteractions(alsoknownasthehydrophobiceffect)arethemajordrivingforcesintheformationoflipidbilayers.Anincreaseininteractionsbetweenhydrophobicmolecules(causingclusteringofhydrophobicregions)allowswatermoleculestobondmorefreelywitheachother,increasingtheentropyofthesystem.ThiscomplexinteractioncanincludenoncovalentinteractionssuchasvanderWaals,electrostaticandhydrogenbonds. Lipidbilayersaregenerallyimpermeabletoionsandpolarmolecules.Thearrangementofhydrophilicheadsandhydrophobictailsofthelipidbilayerpreventpolarsolutes(ex.aminoacids,nucleicacids,carbohydrates,proteins,andions)fromdiffusingacrossthemembrane,butgenerallyallowsforthepassivediffusionofhydrophobicmolecules.Thisaffordsthecelltheabilitytocontrolthemovementofthesesubstancesviatransmembraneproteincomplexessuchaspores,channelsandgates. Flippasesandscramblasesconcentratephosphatidylserine,whichcarriesanegativecharge,ontheinnermembrane.AlongwithNANA,thiscreatesanextrabarriertochargedmoietiesmovingthroughthemembrane. Membranesservediversefunctionsineukaryoticandprokaryoticcells.Oneimportantroleistoregulatethemovementofmaterialsintoandoutofcells.Thephospholipidbilayerstructure(fluidmosaicmodel)withspecificmembraneproteinsaccountsfortheselectivepermeabilityofthemembraneandpassiveandactivetransportmechanisms.Inaddition,membranesinprokaryotesandinthemitochondriaandchloroplastsofeukaryotesfacilitatethesynthesisofATPthroughchemiosmosis.[38] Membranepolarity Seealso:Epithelialpolarity Alphaintercalatedcell Theapicalmembraneofapolarizedcellisthesurfaceoftheplasmamembranethatfacesinwardtothelumen.Thisisparticularlyevidentinepithelialandendothelialcells,butalsodescribesotherpolarizedcells,suchasneurons.Thebasolateralmembraneofapolarizedcellisthesurfaceoftheplasmamembranethatformsitsbasalandlateralsurfaces.Itfacesoutwards,towardstheinterstitium,andawayfromthelumen.Basolateralmembraneisacompoundphrasereferringtotheterms"basal(base)membrane"and"lateral(side)membrane",which,especiallyinepithelialcells,areidenticalincompositionandactivity.Proteins(suchasionchannelsandpumps)arefreetomovefromthebasaltothelateralsurfaceofthecellorviceversainaccordancewiththefluidmosaicmodel.Tightjunctionsjoinepithelialcellsneartheirapicalsurfacetopreventthemigrationofproteinsfromthebasolateralmembranetotheapicalmembrane.Thebasalandlateralsurfacesthusremainroughlyequivalent[clarificationneeded]tooneanother,yetdistinctfromtheapicalsurface. Membranestructures DiagramoftheCellMembrane'sstructures. Cellmembranecanformdifferenttypesof"supramembrane"structuressuchascaveola,postsynapticdensity,podosome,invadopodium,focaladhesion,anddifferenttypesofcelljunctions.Thesestructuresareusuallyresponsibleforcelladhesion,communication,endocytosisandexocytosis.Theycanbevisualizedbyelectronmicroscopyorfluorescencemicroscopy.Theyarecomposedofspecificproteins,suchasintegrinsandcadherins. Cytoskeleton Thecytoskeletonisfoundunderlyingthecellmembraneinthecytoplasmandprovidesascaffoldingformembraneproteinstoanchorto,aswellasformingorganellesthatextendfromthecell.Indeed,cytoskeletalelementsinteractextensivelyandintimatelywiththecellmembrane.[39]Anchoringproteinsrestrictsthemtoaparticularcellsurface—forexample,theapicalsurfaceofepithelialcellsthatlinethevertebrategut—andlimitshowfartheymaydiffusewithinthebilayer.Thecytoskeletonisabletoformappendage-likeorganelles,suchascilia,whicharemicrotubule-basedextensionscoveredbythecellmembrane,andfilopodia,whichareactin-basedextensions.Theseextensionsareensheathedinmembraneandprojectfromthesurfaceofthecellinordertosensetheexternalenvironmentand/ormakecontactwiththesubstrateorothercells.Theapicalsurfacesofepithelialcellsaredensewithactin-basedfinger-likeprojectionsknownasmicrovilli,whichincreasecellsurfaceareaandtherebyincreasetheabsorptionrateofnutrients.Localizeddecouplingofthecytoskeletonandcellmembraneresultsinformationofableb. Intracellularmembranes Thecontentofthecell,insidethecellmembrane,iscomposedofnumerousmembrane-boundorganelles,whichcontributetotheoverallfunctionofthecell.Theorigin,structure,andfunctionofeachorganelleleadstoalargevariationinthecellcompositionduetotheindividualuniquenessassociatedwitheachorganelle. Mitochondriaandchloroplastsareconsideredtohaveevolvedfrombacteria,knownastheendosymbiotictheory.ThistheoryarosefromtheideathatParacoccusandRhodopseudomonas,typesofbacteria,sharesimilarfunctionstomitochondriaandblue-greenalgae,orcyanobacteria,sharesimilarfunctionstochloroplasts.Theendosymbiotictheoryproposesthatthroughthecourseofevolution,aeukaryoticcellengulfedthese2typesofbacteria,leadingtotheformationofmitochondriaandchloroplastsinsideeukaryoticcells.Thisengulfmentleadtothe2membranessystemsoftheseorganellesinwhichtheoutermembraneoriginatedfromthehost'splasmamembraneandtheinnermembranewastheendosymbiont'splasmamembrane.ConsideringthatmitochondriaandchloroplastsbothcontaintheirownDNAisfurthersupportthatbothoftheseorganellesevolvedfromengulfedbacteriathatthrivedinsideaeukaryoticcell.[40] Ineukaryoticcells,thenuclearmembraneseparatesthecontentsofthenucleusfromthecytoplasmofthecell.[41]Thenuclearmembraneisformedbyaninnerandoutermembrane,providingthestrictregulationofmaterialsintoandoutofthenucleus.Materialsmovebetweenthecytosolandthenucleusthroughnuclearporesinthenuclearmembrane.Ifacell'snucleusismoreactiveintranscription,itsmembranewillhavemorepores.Theproteincompositionofthenucleuscanvarygreatlyfromthecytosolasmanyproteinsareunabletocrossthroughporesviadiffusion.Withinthenuclearmembrane,theinnerandoutermembranesvaryinproteincomposition,andonlytheoutermembraneiscontinuouswiththeendoplasmicreticulum(ER)membrane.LiketheER,theoutermembranealsopossessesribosomesresponsibleforproducingandtransportingproteinsintothespacebetweenthetwomembranes.Thenuclearmembranedisassemblesduringtheearlystagesofmitosisandreassemblesinlaterstagesofmitosis.[42] TheER,whichispartoftheendomembranesystem,whichmakesupaverylargeportionofthecell'stotalmembranecontent.TheERisanenclosednetworkoftubulesandsacs,anditsmainfunctionsincludeproteinsynthesis,andlipidmetabolism.Thereare2typesofER,smoothandrough.TheroughERhasribosomesattachedtoitusedforproteinsynthesis,whilethesmoothERisusedmorefortheprocessingoftoxinsandcalciumregulationinthecell.[43] TheGolgiapparatushastwointerconnectedroundGolgicisternae.Compartmentsoftheapparatusformsmultipletubular-reticularnetworksresponsiblefororganization,stackconnectionandcargotransportthatdisplayacontinuousgrape-likestringedvesiclesrangingfrom50-60 nm.Theapparatusconsistsofthreemaincompartments,aflatdisc-shapedcisternawithtubular-reticularnetworksandvesicles.[44] Variations Thecellmembranehasdifferentlipidandproteincompositionsindistincttypesofcellsandmayhavethereforespecificnamesforcertaincelltypes. Sarcolemmainmusclecells:Sarcolemmaisthenamegiventothecellmembraneofmusclecells.[45]Althoughthesarcolemmaissimilartoothercellmembranes,ithasotherfunctionsthatsetitapart.Forinstance,thesarcolemmatransmitssynapticsignals,helpsgenerateactionpotentials,andisveryinvolvedinmusclecontraction.[46]Unlikeothercellmembranes,thesarcolemmamakesupsmallchannelscalledT-tubulesthatpassthroughtheentiretyofmusclecells.Ithasalsobeenfoundthattheaveragesarcolemmais10 nmthickasopposedtothe4 nmthicknessofageneralcellmembrane.[47][45] Oolemmaisthecellmembraneinoocytes:Theoolemmaofoocytes,(immatureeggcells)arenotconsistentwithalipidbilayerastheylackabilayeranddonotconsistoflipids.[48]Rather,thestructurehasaninnerlayer,thefertilizationenvelope,andtheexteriorismadeupofthevitellinelayer,whichismadeupofglycoproteins;however,channelsandproteinsarestillpresentfortheirfunctionsinthemembrane. Axolemma:Thespecializedplasmamembraneontheaxonsofnervecellsthatisresponsibleforthegenerationoftheactionpotential.Itconsistsofagranular,denselypackedlipidbilayerthatworkscloselywiththecytoskeletoncomponentsspectrinandactin.Thesecytoskeletoncomponentsareabletobindtoandinteractwithtransmembraneproteinsintheaxolemma.[49][50] Permeability Seealso:Intestinalpermeability Thepermeabilityofamembraneistherateofpassivediffusionofmoleculesthroughthemembrane.Thesemoleculesareknownaspermeantmolecules.Permeabilitydependsmainlyontheelectricchargeandpolarityofthemoleculeandtoalesserextentthemolarmassofthemolecule.Duetothecellmembrane'shydrophobicnature,smallelectricallyneutralmoleculespassthroughthemembranemoreeasilythancharged,largeones.TheinabilityofchargedmoleculestopassthroughthecellmembraneresultsinpHpartitionofsubstancesthroughoutthefluidcompartmentsofthebody. 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