Ch. 7 notes

Ch. 7 notes - Ch. 7 00:56 Plasma membrane 8 nm thick,...

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Ch. 7 00:56 Plasma membrane – 8 nm thick, exhibits selective permeability (allows some substances to cross it more easily than others) Fluid mosaics of lipids and proteins Phospholipids – amphipathic molecule (has both a hydrophilic and a hydrophobic region) Fluid mosaic model – mosaic of various proteins embedded in or attached to a double layer (bilayer) of phospholipids Sandwich model (Hugh Davson and James Danielli, 1935) Phospholipid bilayer between 2 layers of hydrophilic proteins Problems Membranes with different functions differ in structure and chemical composition Membrane proteins are amphipathic, but in this model, the hydrophobic parts are in aqueous surroundings Fluid mosaic model (S.J. Singer, G. Nicholson, 1972) Membrane proteins reside in the phospholipid bilayer with their hydrophilic regions protruding Maximize contact of hydrophilic regions of proteins and phospholipids with water/extracellular fluid, while providing their hydrophobic parts with a non-aqueous environment Freeze-fracture – method of preparing cells for electron microscopy, splits membrane along middle of bilayer proteins ARE embedded in phospholipid bilayer of the membrane! Fluidity of membranes Lipids, some proteins can shift laterally (10 7 times per second) or flip-flop (rare) At lower temperatures, membrane solidifies Unsaturated hydrocarbon tails (kinked) prevent packing, enhancing membrane fluidity even at low temperatures Saturated hydrocarbon tails pack together, increasing membrane viscosity solidifies sooner *cholesterol: fluidity buffer for membrane, resists changes in membrane fluidity caused by changes in temperature High temperature (37˚C) – makes membrane less fluid by restraining phospholipid movement
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Also hinders the close packing of phospholipids Lowers temperature required for membrane to solidify Evolution – appropriate fluidity of membranes Extreme cold – lots of unsaturated hydrocarbon tails Let membranes stay fluid Extreme heat – lipids that prevent excessive fluidity Varying temperatures – varying changes in fluidity Membrane proteins Integral proteins – penetrate the hydrophobic interior of the lipid bilayer, consist of nonpolar amino acids, usually coiled in α helices, and the hydrophilic parts of the molecule are exposed to aqueous solutions on either side of the membrane Transmembrane proteins span the membrane Peripheral proteins – not embedded in the lipid bilayer at all; they are appendages loosely bound
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This note was uploaded on 03/14/2012 for the course BISC 220 taught by Professor Mcclure during the Spring '09 term at USC.

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Ch. 7 notes - Ch. 7 00:56 Plasma membrane 8 nm thick,...

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