Cell Membranes | Learn Science at Scitable - Nature

Cell membranes protect and organize cells. All cells have an outer plasma membrane that regulates not only what enters the cell, but also how much of any ... Thispagehasbeenarchivedandisnolongerupdated   Aa Aa Aa CellMembranes   Cell membranesprotectandorganizecells.Allcellshaveanouterplasmamembrane thatregulatesnotonlywhatentersthecell,butalsohowmuchofanygiven substancecomesin.Unlikeprokaryotes,eukaryoticcellsalsopossessinternal membranesthatencasetheirorganellesandcontroltheexchangeofessential cellcomponents.Bothtypesofmembraneshaveaspecializedstructurethat facilitatestheirgatekeepingfunction. WhatAreCellularMembranesMadeOf? Withfewexceptions,cellularmembranes—includingplasmamembranesandinternalmembranes—aremadeofglycerophospholipids,moleculescomposedofglycerol,aphosphategroup,andtwofattyacidchains.Glycerolisathree-carbonmoleculethatfunctionsasthebackboneofthesemembranelipids.Withinanindividualglycerophospholipid,fattyacidsareattachedtothefirstandsecondcarbons,andthephosphategroupisattachedtothethirdcarbonoftheglycerolbackbone.Variableheadgroupsareattachedtothephosphate.Space-fillingmodelsofthesemoleculesrevealtheircylindricalshape,ageometrythatallowsglycerophospholipidstoalignside-by-sidetoformbroadsheets(Figure1). Figure1: Thelipidbilayerandthestructureandcompositionofaglycerophospholipidmolecule(A)Theplasmamembraneofacellisabilayerofglycerophospholipidmolecules.(B)Asingleglycerophospholipidmoleculeiscomposedoftwomajorregions:ahydrophilichead(green)andhydrophobictails(purple).(C)Thesubregionsofaglycerophospholipidmolecule;phosphatidylcholineisshownasanexample.Thehydrophilicheadiscomposedofacholinestructure(blue)andaphosphate(orange).Thisheadisconnectedtoaglycerol(green)withtwohydrophobictails(purple)calledfattyacids.(D)Thisviewshowsthespecificatomswithinthevarioussubregionsofthephosphatidylcholinemolecule.Notethatadoublebondbetweentwoofthecarbonatomsinoneofthehydrocarbon(fattyacid)tailscausesaslightkinkonthismolecule,soitappearsbent.©2010NatureEducationAllrightsreserved. FigureDetail Glycerophospholipidsarebyfarthemostabundantlipidsincellmembranes.Likealllipids,theyareinsolubleinwater,buttheiruniquegeometrycausesthemtoaggregateintobilayerswithoutanyenergyinput.Thisisbecausetheyaretwo-facedmolecules,withhydrophilic(water-loving)phosphateheadsandhydrophobic(water-fearing)hydrocarbontailsoffattyacids.Inwater,thesemoleculesspontaneouslyalign—withtheirheadsfacingoutwardandtheirtailsliningupinthebilayer'sinterior.Thus,thehydrophilicheadsoftheglycerophospholipidsinacell'splasmamembranefaceboththewater-basedcytoplasmandtheexteriorofthecell. Altogether,lipidsaccountforabouthalfthemassofcellmembranes.Cholesterolmolecules,althoughlessabundantthanglycerophospholipids,accountforabout20percentofthelipidsinanimalcellplasmamembranes.However,cholesterolisnotpresentinbacterialmembranesormitochondrialmembranes.Also,cholesterolhelpsregulatethestiffnessofmembranes,whileotherlessprominentlipidsplayrolesincellsignalingandcellrecognition. Figure2: Theglycerophospholipidbilayerwithembeddedtransmembraneproteins©2010NatureEducationAllrightsreserved. Inadditiontolipids,membranesareloadedwithproteins.Infact,proteinsaccountforroughlyhalfthemassofmostcellularmembranes.Manyoftheseproteinsareembeddedintothemembraneandstickoutonbothsides;thesearecalledtransmembraneproteins.Theportionsoftheseproteinsthatarenestedamidthehydrocarbontailshavehydrophobicsurfacecharacteristics,andthepartsthatstickoutarehydrophilic(Figure2). Atphysiologicaltemperatures,cellmembranesarefluid;atcoolertemperatures,theybecomegel-like.Scientistswhomodelmembranestructureanddynamicsdescribethemembraneasafluidmosaicinwhichtransmembraneproteinscanmovelaterallyinthelipidbilayer.Therefore,thecollectionoflipidsandproteinsthatmakeupacellularmembranereliesonnaturalbiophysicalpropertiestoformandfunction.Inlivingcells,however,manyproteinsarenotfreetomove.Theyareoftenanchoredinplacewithinthemembranebytetherstoproteinsoutsidethecell,cytoskeletalelementsinsidethecell,orboth. WhatDoMembranesDo? Cellmembranesserveasbarriersandgatekeepers.Theyaresemi-permeable,whichmeansthatsomemoleculescandiffuseacrossthelipidbilayerbutotherscannot.Smallhydrophobicmoleculesandgaseslikeoxygenandcarbondioxidecrossmembranesrapidly.Smallpolarmolecules,suchaswaterandethanol,canalsopassthroughmembranes,buttheydosomoreslowly.Ontheotherhand,cellmembranesrestrictdiffusionofhighlychargedmolecules,suchasions,andlargemolecules,suchassugarsandaminoacids.Thepassageofthesemoleculesreliesonspecifictransportproteinsembeddedinthemembrane. Figure3: SelectivetransportSpecializedproteinsinthecellmembraneregulatetheconcentrationofspecificmoleculesinsidethecell.©2010NatureEducationAllrightsreserved. Membranetransportproteinsarespecificandselectiveforthemoleculestheymove,andtheyoftenuseenergytocatalyzepassage.Also,theseproteinstransportsomenutrientsagainsttheconcentrationgradient,whichrequiresadditionalenergy.Theabilitytomaintainconcentrationgradientsandsometimesmovematerialsagainstthemisvitaltocellhealthandmaintenance.Thankstomembranebarriersandtransportproteins,thecellcanaccumulatenutrientsinhigherconcentrationsthanexistintheenvironmentand,conversely,disposeofwasteproducts(Figure3). Othertransmembraneproteinshavecommunication-relatedjobs.Theseproteinsbindsignals,suchashormonesorimmunemediators,totheirextracellularportions.Bindingcausesaconformationalchangeintheproteinthattransmitsasignaltointracellularmessengermolecules.Liketransportproteins,receptorproteinsarespecificandselectiveforthemoleculestheybind(Figure4). Figure4: ExamplesoftheactionoftransmembraneproteinsTransporterscarryamolecule(suchasglucose)fromonesideoftheplasmamembranetotheother.Receptorscanbindanextracellularmolecule(triangle),andthisactivatesanintracellularprocess.Enzymesinthemembranecandothesamethingtheydointhecytoplasmofacell:transformamoleculeintoanotherform.Anchorproteinscanphysicallylinkintracellularstructureswithextracellularstructures.©2010NatureEducationAllrightsreserved. FigureDetail Peripheralmembraneproteinsareassociatedwiththemembranebutarenotinsertedintothebilayer.Rather,theyareusuallyboundtootherproteinsinthemembrane.Someperipheralproteinsformafilamentousnetworkjustunderthemembranethatprovidesattachmentsitesfortransmembraneproteins.Otherperipheralproteinsaresecretedbythecellandformanextracellularmatrixthatfunctionsincellrecognition. HowDiverseAreCellMembranes? In contrasttoprokaryotes,eukaryoticcellshavenotonlyaplasma membranethat encasestheentirecell,butalsointracellularmembranesthatsurroundvarious organelles.Insuchcells,theplasmamembraneispartofanextensiveendomembrane systemthatincludesthe endoplasmicreticulum(ER),thenuclearmembrane,theGolgiapparatus, and lysosomes.Membranecomponentsareexchangedthroughouttheendomembrane system inanorganizedfashion.Forinstance,themembranesoftheERandthe Golgi apparatushavedifferentcompositions,andtheproteinsthatarefound inthese membranescontainsortingsignals,whicharelikemolecularzipcodes that specifytheirfinaldestination. Mitochondria andchloroplastsarealsosurroundedbymembranes,buttheyhaveunusual membranestructures—specifically,eachoftheseorganelleshastwo surrounding membranesinsteadofjustone.Theoutermembraneofmitochondriaand chloroplastshasporesthatallowsmallmoleculestopasseasily.The inner membraneisloadedwiththeproteinsthatmakeuptheelectrontransport chain andhelpgenerateenergyforthecell.Thedoublemembraneenclosuresof mitochondriaandchloroplastsaresimilartocertainmodern-day prokaryotesand arethoughttoreflecttheseorganelles'evolutionaryorigins. Conclusion Membranesaremadeoflipidsandproteins,andthey serveavarietyofbarrierfunctionsforcellsandintracellular organelles. Membraneskeeptheoutside"out"andtheinside"in,"allowingonly certain moleculestocrossandrelayingmessagesviaachainofmolecularevents eBooks ThispageappearsinthefollowingeBook EssentialsofCellBiology,Unit3.1 CellBiologyforSeminars,Unit3.1 TopicroomswithinCellBiology Close Notopicroomsarethere. Loading... 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