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Chapter 9 . Membranes and Membrane TransportChapter 9Membranes and Membrane Transport. . . . . . . . . . . . . . . . . . . . . . . .Chapter OutlineMembrane functions:Boundary for cell and organelleSurface on which reactions can occurRegulation of material flux through membrane proteinsSignal transduction interfaceSpecialized properties: Photosynthesis, electron transport, electrical activityMembrane componentsLipids: Amphipathic molecules arranged as bilayers with polar groups out and nonpolar groups inProteins: Surface associated or embedded in bilayerPlasma membraneDelimits cellExcludes and retains certain ions and moleculesMajor role in energy transductionCell locomotionReproductionSignal transductionInteractions with other cells or extracellular matrixLipid interactionsMonolayers: Formation of single-molecule-thick layer at air/water interface with polar groups in contact with waterMicelles: Lipid spheres with polar groups out and hydrophobic tails in the center: Critical micelle concentration is the concentration of amphiphilic compound at which micelles formLipid bilayer: Two lipid monolayers with hydrophobic surfaces face to faceLiposomes: Vesicles formed by lipid bilayersFluid mosaic modelSinger and Nicholson, 1972Phospholipid bilayer forming fluid matrixTwo classes of membrane proteinsPeripheral (extrinsic) proteinsAssociated with bilayer surface via ionic interactions and H bondsExtractable with high salt or agents that disrupt H bonds (urea)Integral (intrinsic) proteinsAssociate with hydrophobic bilayer interior via hydrophobic interactionsExtractable with detergentsMembrane mobility126
Chapter 9 . Membranes and Membrane TransportProteinFrye and Edidin, 1970: Lateral movement of membrane proteins following fusion of mouse and human cellsLateral movement may be impeded by interactions with cytoskeletonLipidsRapid lateral movementSlow transverse movementMembrane asymmetryLateral asymmetry arises from clustering of membrane components within the planeLipid clustering: Phase separation induced by divalent cations and influenced by lipid typeProtein clustering: Self-associating membrane proteins e.g., bacteriorhodopsinTransverse asymmetryLipids: Lipid asymmetry due to two processesAsymmetric synthesisEnergy-dependent transport: FlippasesProteins: Asymmetric moleculesCarbohydrates: Glycoproteins and glycolipids on outer surfaceMembrane phase transitions: Radical change in physical state occurring within narrow range of transition (or melting) temperature. Below TmLipids close-pack: Lose lateral mobility and rotational mobility of fatty acid chainsConsequences: Membrane thickens and decreases surface areaCharacteristicsTm increases with chain length degree of saturation and is influenced by nature of head groupPure phospholipid bilayers show narrow temperature range