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1、 Thestructureandfunctionoftheplasmamembrane Learningobjectives 1 Modelofmembranestructure 2 Modelofmembranestructure anexperimentalperspective 3 Thechemicalcompositionofmembranes4 Characteristicsofmembrane 5 Anoverviewofthefunctionsofmembranes 1 Abriefhistoryofstudiesonplasmamembranestructure A Conc
2、epts Plasmamembrane cellmembrane Intracellular cytoplasmic membrane Biomembrane B Thehistoryofstudy Overton 1890s LipidnatureofPM GorterandGrendel 1925 Thebasisofmembranestructureisalipidbilayer Toanswerthequestionthathowmanylipidlayerswereinmembrane in1925GorterandGrendelextractedthelipidsfromaknow
3、nnumberoferythrocytesandspreadthelipidfilmonawatersurface Theareaoflipidfilmonthewaterwasabouttwice 1 8 2 2 theestimatedtotalsurfaceareaoftheerythrocytes sotheyconcludedthattheerythrocyteplasmamembraneconsistedofnotonebuttwolayersoflipids H DavsonandJ Danielli 1935 1954 sandwichmodel Membranesalsoco
4、ntainproteins Ifthemembranesonlyconsistofpurelipids itcouldnotexplainallthepropertiesofmembranes Forexample sugars ions andotherhydrophilicsolutesmoveintoandoutofcellsmuchmorereadilythancouldbeexplainedbythepermeabilityofpurelipidbilayers Toexplainsuchdifferences DavsonandDanielliinvokedthepresenceo
5、fproteinsinmembranesin1935 Theplasmamembraneiscomposedofalipidbilayerthatislinedonbothitsinnerandoutersurfacebyalayerofglobularproteins inadditionto thepresenceofprotein linedporesforpolarsolutesandionstoenterandexitthecell J D Robertson 1959 TheTEMshowing thetrilaminarappearanceofPM Unitmembranemod
6、el S J SingerandG Nicolson 1972 fluid mosaicmodel SingerandNicolson sModelofmembranestructure Thefluid mosaicmodelisthe centraldogma ofmembranebiology A Thecorelipidbilayerexistsinafluidstate capableofdynamicmovement B Membraneproteinsformamosaicofparticlespenetratingthelipidtovaryingdegrees TheFlui
7、dMosaicModel proposedin1972bySingerandNicolson hadtwokeyfeatures bothimpliedinitsname Experimentalevidenceforthe mosaic propertyofmembranes Freeze fracturethemembraneandobserveprotein bumps K Simonsetal 1997 lipidraftsmodel FunctionalraftsinCellmembranes Nature387 569 572 Atomicforcemicroscopyreveal
8、ssphyingomyelinrafts orange protrudingfromadioleoylphosphatidylcholinebackground black inamica supportedlipidbilayer Placentalalkalinephosphatase yellowpeaks aglycosylphosphatidylinositol anchoredprotein isshowntobealmostexclusivelyraftassociated FromD E Saslowskyetal 2002 J Biol Chem 277 26966 2697
9、0 2 Thechemicalcompositionofmembranes zMembranelipid thelipidbilayerservesprimarilyasastructuralbackboneofthemembraneandprovidesthebarrierpreventingtherandommovementofwater solublematerialsintoandoutofcell zMembraneproteins carryoutmostofthe specificfunctions zCarbohydrates havebiologicalfunctions s
10、uchas protection recognition adhesion Thebasiccompositionsofsomebio membranes MembranePlasmamembraneRedbloodcell Proteins Lipids Saccharide 491854666462 437936322627 83 MyelinmembraneLivercell 102 NucleusmembraneGolgibody 1010 EndoplasmicreticulumMitochondrionOutsidemembraneInsidemembraneChloroplast
11、 557870 452230 trace A MembraneLipids TheFluidPartoftheModel Membranelipidsareamphipathic hydrophilic hydrophobic Therearethreemajorclassesofmembranelipids zPhospholipids PhosphoglycerideandsphingolipidszGlycolipids zSterols isonlyfoundinanimals Figure10 2 Thepartsofaphospholipidmolecule Phosphatidy
12、lcholine representedschematically A informula B asaspace fillingmodel C andasasymbol D Thekinkduetothecis doublebondisexaggeratedinthesedrawingsforemphasis 鞘氨醇 Theclassofsphingolipids 神经酰胺鞘磷脂 神经节苷脂 Effectoflipidcompositiononbilayerthicknessandcurvature a Apuresphingomyelin SM bilayeristhickerthanone
13、formedfromaphosphoglyceridesuchasphosphatidylcholine PC Cholesterolhasalipid orderingeffectonphosphoglyceridebilayersthatincreasestheirthicknessbutdoesnotaffectthethicknessofthemoreorderedSMbilayer b PhospholipidssuchasPChaveacylindricalshapeandformmoreorlessflatmonolayers whereasthosewithsmallerhea
14、dgroupssuchas phosphatidylethanolamine PE haveaconicalshape c AbilayerenrichedwithPCintheexoplasmicleafletandwithPEinthecytosolicface asinmanyplasmamembranes wouldhaveanatural curvature AdaptedfromH Sprongetal 2001 NatureRev Mol CellBiol 2 504 Glycolipidmolecules Galactocerebroside A iscalledaneutra
15、lglycolipidbecausethesugarthatformsitsheadgroupisuncharged Aganglioside B alwayscontainsoneormorenegativelychargedsialicacidresidues alsocalledN acetylneuraminicacid orNANA whosestructureisshownin C Whereasinbacteriaandplantsalmostallglycolipidsarederivedfromglycerol asaremostphospholipids inanimalc
16、ellstheyarealmostalwaysproducedfromsphingosine anaminoalcoholderivedfromserine asisthecaseforthephospholipidsphingomyelin Gal galactose Glc glucose GalNAc N acetylgalactos amine thesethreesugarsareuncharged Thestructureofcholesterol Cholesterolisrepresentedbyaformulain A byaschematicdrawingin B andasaspace fillingmodelin C Themovementofmembranelipid Fourkindsofmovement Lateraldiffusionbyexchangingplaces Rotationarounditslongaxis Wave Transversediffusion or flip flop fromonemonolayertotheother Fl
