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Chapter 5

Chapter 5 - Membranes and Transport.docx

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BIOL 204
Dr.Roland Treu

CHAPTER 5 MEMBRANES AND TRANSPORT51 An Overview of the Structure of MembranesPlasma membraneselectively permeable barrier allows for uptake of key nutrients and elimination of wasteproducts while maintaining protected env in which metabolic processes can occur51a The Fluid Mosaic Model of MembranesMembranes are not rigid w mol locked in placed but rather consist of fluid lipid bilayer in which proteins are embedded and float freelyoRarely does lipid mol flipflop btwn 2 layersoMosaic refers to fact that membranes contain wide assortment of diff types of proteins each w nspecific fSmallof membrane proteins anchor cytoskeleton filaments to mem and thus dN moveRelative proportions of lipidprotein win mem vary depending on type of membranensBilayer is asymmetrical diff in f performed by each side51b Experimental Evidence in Support of the Fluid Mosaic Model1Membranes are fluidoFryeEdidin expmt mem proteins found to rapidly migrate over surface of hybrid cell seen by dye2Membrane asymmetryTMoFreezefractureanalyzed split bilayer images show particles on either side differing in sizeand shape 52 The Lipid Fabric of a Membrane52a Phospholipids Are the Dominant Lipids in MembranesPhospholipids consist of 2 fatty acid tails linked to 1 AA by a phosphate groupAll are amphipathiceach phospholipid mol contains hydrophobichydrophilic regionoFatty acid chain is hydrophobic nonpolaroPhosphatecontaining head group is hydrophilic polarnWhen added to aqueous sol phospholipids associate w each other and assemble into a bilayeroNo reqmts reqdspontaneous 52b Membrane Fluidityd on how densely the indv lipid mol can pack tgh influenced by1Composition of lipid mol tht make up memFatty acids composed of saturated hydrocarbons tend to have straight shape allowing for tighter packingUnsaturated fatty acids less straight as db bonds introduce kinksbends in structure resulting looser packing2TempDrops low enough closely packed and mem forms highly viscous semisolid gelHigh enough mem too fluid and liquid and can result in mem leakage52c Organisms Can Adjust Fatty Acid CompositionEx for organisms in extreme cold envoBy regulating amnt of desaturase enzyme synthesized can closely regulate amnt of unsaturated fatty acids incorporated into membranes and thereby maintain membrane fluidity win optimum rangeSterols also influence membrane fluidityeg cholesteroloHigh temp restrain mvmt of lipid mol thus reducing fluidity of memoLow temp disrupt fatty acids from associating by occupying space btwn lipid mol thus slowing transition to nonfluid gel state53 Membrane ProteinsAlthough lipid mol constitute the backbone of a mem the unique set of proteins associated w mem dtrmns nits f and makes each mem unique53a The Key Functions of Membrane Proteins1Transport
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