Membranes Transport

Membranes Transport - 1 Transport Across Biological...

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Pernecky A. Biological Membranes 1. The lipid bilayer fig. 11-4 a. Phospholipid headgroup: Inward and outward from cell ---> H 2 O b. Fatty acyl tails directed toward inside the membrane ---> nonpolar 2. Consituents of membranes: Asymmetric grouping of proteins and lipids fig. 11-3 a. Membrane proteins (1) Integral or intrinsic proteins : Tightly bound to membranes by hydrophobic forces (detergents, organic solvents needed to remove). These proteins are amphiphilic (amphipathic). Transmembrane segments span the membrane and have a high content of apolar amino acids. (2) Peripheral or extrinsic proteins : Dissociated from membranes with mild procedures (increase in ionic strength, pH, metal chelating agents). b. Assymetry in membranes fig. 11-5 (1) Lipids (RBCs, or example) (a) Outer leaflet: Sphingomyelin, phosphatidylcholine, glycolipids (b) Inner leaflet: Phosphatidylethanolamine, phosphatidylserine (2) Proteins (a) Glycoproteins (receptors): Outer leaflet (b) Second messenger systems: Inner leaflet 3. The fluid mosaic model of membrane structure (Singer and Nicholson) a. Free diffusion of proteins within a "sea" of lipid. fig. 11-7 i. Unsaturated lipids contribute to the fluidity ii. Cholesterol decreases fluidity (but prevents close packing of fatty acids at low temperatures). b. Lipids have thermal motion, will move laterally, and occasionally move between outer and inner leaf by uncatalyzed (flip-flop) or enzyme-catalyed (flippase) movement. fig. 11-16 c. The membrane is a heterogeneous mixture of glycoproteins, glycolipids, lipids, proteins. i. Integral membrane proteins frequently span the membrane, have highly apolar regions that interact with membrane lipids by hydrophobic forces, have a certain orientation in the membrane, and are removed only with detergents fig. 11-7 ii. Peripheral membrane proteins are held to membrane by electrostatic interactions, and can be removed by treatment with chelating agents or a change in pH. fig. 11-6 d. Some proteins are held in place in the membrane by protein scaffolding, whereas others are covalently linked to lipid. fig. 11-19 e. Hydropathy plots predict membrane spanning regions in integral membrane proteins. The relative hydrophobicity/hydrophilicity for each amino acid position within a “window” is computed and plotted – positive values indicate hydrophobic regions. Usually, alpha helical regions, but can also be β -sheets. fig. 11-11
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Membranes Transport - 1 Transport Across Biological...

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