CellBiology2.4CellMembrane

Essen5alQues5on:Thestructureofbiologicalmembranesmakethemfluidanddynamic

Function of the Cell Membrane •  Hold the cell together

•  Isolate the cytoplasm and organelles from external environment

•  Protect the cell •  Regulate the exchange of

substances •  Control what goes in and out

•  To bind to other cells and molecules

•  Cell identification •  Allow cell to be recognized and to

recognize (cell signaling, immunity)

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Structure of the Cell Membrane

Phospholipids Primary structural building block of the plasma membrane

Hydrophobic hydrocarbon tails

Water repelled

Hydrophilic phosphate heads

Water attracted

Amphipathic Phospholipids contain both

hydrophilic (water-loving) and lipophilic (fat-loving) regions.

Phospholipids Arrangement in Membranes

•  Spontaneouslyarrangesintoabilayer•  Bilayerisheldtogetherbyweakhydrophobicinterac5ons

betweenthetails•  Amphipathiclayersrestrictthepassageofmanysubstances•  Individualphospholipidscanmovewithinthebilayercreate

fluidityandflexibility•  Allowsforspontaneousbreakingandreformingofmembrane

Cholesterol(eg.Lipoprotein)Animportantstructuralelementofanimalcellmembranes

•  Providesstrengthandstabilitytothemembranestructure•  Makesmembranelessfluidbybindingtogetherthelipidinthe

plasmamembrane

Glycolipid•  PhospholipidwithaCHOaNached•  Provideenergy•  Cellularrecogni5onmarkersforspecificelements

Proteins•  Servemanyfunc5onsdependingonstructureandloca5on

•  Maybeintegral(embedded)orperipheral(ontheinsidesurfaceofmembrane)

-Theseproteinscansitononeofthesurfaces.-Theyslidearoundthemembranequicklyandcollidewithoneanother.-Theycanneverflipfromonesidetotheother.-Involvedinmaintainingcellshape,orcellmo5lity.Mayalsobeenzymes,catalysingreac5onsincytoplasm

Theseproteinscanspanfromonesideofthebilayertotheother.Becausetheyspanthemembrane,theyareoUeninvolvedinthetransportofsubstancesacrossthemembrane.

Glycoprotein•  ProteinwithaCHOa1ached•  Importantforcell-cellinterac5ons•  Importantforwhitebloodcellrecogni5on•  Actasreceptorsincellsignaling(suchaswithhormones)

•  wasbasicallya"sandwich"oflipids(arrangedinabilayer)coveredonbothsideswithproteins

•  thismodelisNOTcorrectbutshowstheprogressionofthecurrentmodel

•  oneofthefirstmodelsofmembranetobeacceptedbybiologistsformanyyears.

Davson–DanielliModel(1930’s)

ModelsoftheCellMembrane

CellCytoplasm

ExtracellularEnvironment

•  S.J.SingerandNicolson(1966)•  Semipermeablemembrane•  “fluid”por5onisdoublelayerofphospholipids

(=phospholipidbilayer)•  “mosaic”por5onistheproteinsandcarbohydrates

embeddedinthemembrane

FluidMosiacModel