chapter7 - Chapter 7: Membranes I. BIOL 1020 CHAPTER 7...

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BIOL 1020 – CHAPTER 7 LECTURE NOTES Chapter 7: Membranes I. Roles of biological membranes A. membranes separate aqueous environments, so that differences can be maintained 1. the plasma membrane surrounds the cell and separates the interior of the cell from the external environment 2. membrane-bound organelles have their interior region separated from the rest of the cell B. passage of substances across membranes is generally regulated, helping to establish and maintain appropriate environments in the cell even as the outside environment changes C. membranes provide a surface on which many chemical events can occur 1. enzymes embedded in membranes catalyze many chemical reactions, and the locations of reactants and products on one side or the other of the membrane is often used to help control reaction rates 2. proteins and glycoproteins embedded in membranes are used for chemical recognition and signaling II. Physical properties of cell membranes: the lipid bilayer and the fluid mosaic model A. biological membranes are lipid bilayers with associated proteins and glycoproteins 1. most of the lipids involved are phospholipids, although others like cholesterol and various glycolipids are also present B. phospholipids molecules spontaneously form bilayers in aqueous environments due to their amphipathic nature and overall cylindrical structure 1. amphipathic molecules have distinct hydrophobic and hydrophilic regions 2. recall the hydrophilic “head” and hydrophobic “tails” of phospholipids tails come from two chains of fatty acids linked to glycerol head comes from a polar organic molecule linked via a phosphate group to the glycerol backbone 3. the two tails combine with the head to give a roughly cylindrical shape to the phospholipids molecule, a shape that favors the formation of lipid bilayers over lipid spheres 4. there are other amphipathic molecules, such as detergents (soaps, etc.), that come to a point at their single hydrophobic tail, thus tending to form spheres instead of bilayers 5. detergents can “solubilize” lipids to varying degrees; high enough concentrations of detergents will disrupt cell membranes C. the fluid mosaic model describes the structure and properties of cell membranes 1. while a structural model including a lipid bilayer was proposed in the 1930s, early models sandwiched the lipid bilayer with membrane-associated proteins 2. EM data after the 1950s showed that membrane bilayers are uniformly about 8 nm thick, too thin for the sandwich model; also, isolated membrane proteins were often found to have a globular nature that did not fit the sandwich model 3. in 1972, the fluid mosaic model was proposed where some proteins are imbedded in lipid bilayers that act as two- dimensional fluids; this model explained the existing data and made two key predications that have been verified: materials, including embedded proteins, can be moved along the membrane due to its fluid properties digestion of certain “transmembrane” proteins applied to one side of a membrane will produce protein fragments
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This note was uploaded on 12/05/2011 for the course BIOL 1020 taught by Professor Dute during the Fall '06 term at Auburn University.

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chapter7 - Chapter 7: Membranes I. BIOL 1020 CHAPTER 7...

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