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Chapter 11

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CHY 204
Mario Estable

CHAPTER 11: BIOLOGICAL MEMBRANES & TRANSPORT -Membranes define the external boundaries of cells & regulate the molec traffic across that boundary. In eukaryotic cells, memnranes divide the internal space into discrete compartments to segregate processes & components; they organize complex rxn seq that are central to biological energy conservation & cell-cell comm. Membranes are flexible, self-healing & selectively permeable to polar solutes; their flexibility permits shape changes that accompany cell growth & movement/ with their ability reseal & break, 2 membranes can fuse to yield a single membrane enclosed compartment or undergo fission to yield 2 sealed compartments. There are cell membranes, organelle membranes w/ thin lipid bilayers. Integral trans-membrane & surface pro prmote/catalyze various cellular processes: transporters, at the cell surface move specific organic solutes & inorganic ions across the membrane; receptors sense extracellular signals & trigger molecular changes in the cell; adhesion pro hold neighbouring molec together. b/c membranes consist of 2 thin layers which are 2D; intramollecular collisions are far more likely in 2D space so eff of enzyme-catalyzed processes organize w.in membranes are vastly inc. Biological membranes have a trilaminar appearance, 5-8A thick; 1 nm=10^-9 m & 1 A =10^-10 -Composition of membranes are tailored to their functions: the relative proportions of pro & lipid may vary w/in the type of membrain, eg. certain neurons have a myelin sheath which acts as a passive electrical insulator; consists primarily of lipids; bacterial plasma membranes contain more lipids than pro, enriched in cholesterol and involve in metabolic rxns; chloroplast membranes are involved in photosynthesis & mitochondrial membranes are also involved in metabolic rxns. -Fluid mosaic model of membranes: the fatty acyl chains in the interior of the membrane form a fluid, hydrophobic region. Integral pro float in the sea of lipids held by hydrophobic interactions w/ their nonpolar aa side chains. Both pro & lipids are free to move laterally in the plane of the bilayer b/c are mostly held by noncovalent interactions but movement of either from one leaflet of bilayer to another is restricted. CHO attached to some pro & lipids of the pm are exposed to the cellular surface of the membranes. Orientation of pro in the bilayer is asymmetric giving the membrane sidedness. -Cell membranes allow for cell-cell adhesion, receptor-ligand interaction via signal transduction & membrane fusion & consists of transporters such as aquaporines—H2O transporters. Depending on the precise conditions & nature of lipids, 3 types of lipid aggregate can form when ampiphatic lipids are mixed w/ H2O: micelles (spherical structures containing 20-1000 molec; hydrophobic chains of FA are sequestered at the core of the sphere & there is virtually no H2O in the hydrophobic interior; favoured when cross-sectional area of head group is greater than that of the acyl side chain/s, eg. SDS; 1 tail micelles are wedged shaped. In an open bilayer, all acyl side chains except those at the edges of the sheet are protected from interation w/ H2O; individual units are cyndrical & favoured if cross section of head equals that of side chain, eg. glycerophospholipids, sphingolipids & sterols. B/c the hydrophobic regions at the edges are in contact w/ H2O, the lipid bilayer is unstable & spontaneously folds back on itself to form a hollow sphere, vesicle; has a continuous surface tha eliminates exposed hydrophobic regions allowing bilayers to achieve maximal stability in their aq environment; aka liposome & usually implicated in pharmaceutical drug-delivery. -Why are there more unsat FA when grown at lower temp? cells regulate their lipid composition to achieve a constant membrane fluidity under various growth, conditions, eg. bacteria synthesizes more unsat FA & fewer sat FA when cultured at low temps; as a result of this adjustment in lipid composition, membranes of bacteria cultured at high/low temp have about the same degree of fluidity. -Integral membrane pro are very firmly assoc w/ the lipid bilayer & are removable only by agents that interfere w/ hydrophobic interactions, eg. detergents, organic solvents or denaturants. Peripheral pro assoc w/ the membrane through electrostatic interactions & H bonding w/ hydrophilic domains of integral pro & w/ polar head groups of membrane lipids; can be released by mild treatments that interrupt electrostatic interactions or break H bonds, eg. carbonate at high pH. Amphitropic pro are found both in cytosol & in assocn w/ membranes, affinity for membranes result from
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