5.2: Components and Structure - Fluid Mosaic Model

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, ... Skiptomaincontent LearningObjectivesKeyPointsKeyTerms LearningObjectives Describethefluidmosaicmodelofcellmembranes ThefluidmosaicmodelwasfirstproposedbyS.J.SingerandGarthL.Nicolsonin1972toexplainthestructureoftheplasmamembrane.Themodelhasevolvedsomewhatovertime,butitstillbestaccountsforthestructureandfunctionsoftheplasmamembraneaswenowunderstandthem.Thefluidmosaicmodeldescribesthestructureoftheplasmamembraneasamosaicofcomponents—includingphospholipids,cholesterol,proteins,andcarbohydrates—thatgivesthemembraneafluidcharacter.Plasmamembranesrangefrom5to10nminthickness.Forcomparison,humanredbloodcells,visiblevialightmicroscopy,areapproximately8µmwide,orapproximately1,000timeswiderthanaplasmamembrane.Theproportionsofproteins,lipids,andcarbohydratesintheplasmamembranevarywithcelltype.Forexample,myelincontains18%proteinand76%lipid.Themitochondrialinnermembranecontains76%proteinand24%lipid. Figure\(\PageIndex{1}\):TheComponentsandfunctionsofthePlasmaMembrane:Theprincipalcomponentsofaplasmamembranearelipids(phospholipidsandcholesterol),proteins,andcarbohydratesattachedtosomeofthelipidsandsomeoftheproteins. Figure\(\PageIndex{1}\):Thefluidmosaicmodeloftheplasmamembrane:Thefluidmosaicmodeloftheplasmamembranedescribestheplasmamembraneasafluidcombinationofphospholipids,cholesterol,andproteins.Carbohydratesattachedtolipids(glycolipids)andtoproteins(glycoproteins)extendfromtheoutward-facingsurfaceofthemembrane. Themainfabricofthemembraneiscomposedofamphiphilicordual-loving,phospholipidmolecules.Thehydrophilicorwater-lovingareasofthesemoleculesareincontactwiththeaqueousfluidbothinsideandoutsidethecell.Hydrophobic,orwater-hatingmolecules,tendtobenon-polar.Aphospholipidmoleculeconsistsofathree-carbonglycerolbackbonewithtwofattyacidmoleculesattachedtocarbons1and2,andaphosphate-containinggroupattachedtothethirdcarbon.Thisarrangementgivestheoverallmoleculeanareadescribedasitshead(thephosphate-containinggroup),whichhasapolarcharacterornegativecharge,andanareacalledthetail(thefattyacids),whichhasnocharge.Theyinteractwithothernon-polarmoleculesinchemicalreactions,butgenerallydonotinteractwithpolarmolecules.Whenplacedinwater,hydrophobicmoleculestendtoformaballorcluster.Thehydrophilicregionsofthephospholipidstendtoformhydrogenbondswithwaterandotherpolarmoleculesonboththeexteriorandinteriorofthecell.Thus,themembranesurfacesthatfacetheinteriorandexteriorofthecellarehydrophilic.Incontrast,themiddleofthecellmembraneishydrophobicandwillnotinteractwithwater.Therefore,phospholipidsformanexcellentlipidbilayercellmembranethatseparatesfluidwithinthecellfromthefluidoutsideofthecell. Figure\(\PageIndex{1}\):Phospholipidaggregation:Inanaqueoussolution,phospholipidstendtoarrangethemselveswiththeirpolarheadsfacingoutwardandtheirhydrophobictailsfacinginward. Figure\(\PageIndex{1}\):Thestructureofaphospholipidmolecule:Thisphospholipidmoleculeiscomposedofahydrophilicheadandtwohydrophobictails.Thehydrophilicheadgroupconsistsofaphosphate-containinggroupattachedtoaglycerolmolecule.Thehydrophobictails,eachcontainingeitherasaturatedoranunsaturatedfattyacid,arelonghydrocarbonchains. Proteinsmakeupthesecondmajorcomponentofplasmamembranes.Integralproteins(somespecializedtypesarecalledintegrins)are,astheirnamesuggests,integratedcompletelyintothemembranestructure,andtheirhydrophobicmembrane-spanningregionsinteractwiththehydrophobicregionofthethephospholipidbilayer.Single-passintegralmembraneproteinsusuallyhaveahydrophobictransmembranesegmentthatconsistsof20–25aminoacids.Somespanonlypartofthemembrane—associatingwithasinglelayer—whileothersstretchfromonesideofthemembranetotheother,andareexposedoneitherside.Somecomplexproteinsarecomposedofupto12segmentsofasingleprotein,whichareextensivelyfoldedandembeddedinthemembrane.Thistypeofproteinhasahydrophilicregionorregions,andoneorseveralmildlyhydrophobicregions.Thisarrangementofregionsoftheproteintendstoorienttheproteinalongsidethephospholipids,withthehydrophobicregionoftheproteinadjacenttothetailsofthephospholipidsandthehydrophilicregionorregionsoftheproteinprotrudingfromthemembraneandincontactwiththecytosolorextracellularfluid. Figure\(\PageIndex{1}\):Structureofintegralmembraneproteins:Integralmembraneproteinsmayhaveoneormorealpha-helicesthatspanthemembrane(examples1and2),ortheymayhavebeta-sheetsthatspanthemembrane(example3). Carbohydratesarethethirdmajorcomponentofplasmamembranes.Theyarealwaysfoundontheexteriorsurfaceofcellsandareboundeithertoproteins(formingglycoproteins)ortolipids(formingglycolipids).Thesecarbohydratechainsmayconsistof2–60monosaccharideunitsandcanbeeitherstraightorbranched.Alongwithperipheralproteins,carbohydratesformspecializedsitesonthecellsurfacethatallowcellstorecognizeeachother.Thisrecognitionfunctionisveryimportanttocells,asitallowstheimmunesystemtodifferentiatebetweenbodycells(called“self”)andforeigncellsortissues(called“non-self”).Similartypesofglycoproteinsandglycolipidsarefoundonthesurfacesofvirusesandmaychangefrequently,preventingimmunecellsfromrecognizingandattackingthem.Thesecarbohydratesontheexteriorsurfaceofthecell—thecarbohydratecomponentsofbothglycoproteinsandglycolipids—arecollectivelyreferredtoastheglycocalyx(meaning“sugarcoating”).Theglycocalyxishighlyhydrophilicandattractslargeamountsofwatertothesurfaceofthecell.Thisaidsintheinteractionofthecellwithitswateryenvironmentandinthecell’sabilitytoobtainsubstancesdissolvedinthewater. KeyPoints Themainfabricofthemembraneiscomposedofamphiphilicordual-loving,phospholipidmolecules. Integralproteins,thesecondmajorcomponentofplasmamembranes,areintegratedcompletelyintothemembranestructurewiththeirhydrophobicmembrane-spanningregionsinteractingwiththehydrophobicregionofthephospholipidbilayer. Carbohydrates,thethirdmajorcomponentofplasmamembranes,arealwaysfoundontheexteriorsurfaceofcellswheretheyareboundeithertoproteins(formingglycoproteins)ortolipids(formingglycolipids). KeyTerms amphiphilic:Havingonesurfaceconsistingofhydrophilicaminoacidsandtheoppositesurfaceconsistingofhydrophobic(orlipophilic)ones. hydrophilic:Havinganaffinityforwater;abletoabsorb,orbewettedbywater,“water-loving.” hydrophobic:Lackinganaffinityforwater;unabletoabsorb,orbewettedbywater,“water-fearing.”