bio- chap 8 - I Membrane Structure A History ans Components...

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I. Membrane Structure A. History ans Components 1. 1895: Charles Overton postulates membranes are made of lipids a. based on observations that substances that didsolve in lipids enter cells much more rapidly than subs. That are insoluble in lipids 2. 1915: membranes from RBCs are chemically analyzed and found to be composed of lipids and proteins a. phopholipids are more abundant lipids in most membs. b. Ability of phospholipids to form membs. Is part of structure. Molecules in amphipathic (both hydrophilic and hydrophobic region) 3. 1917: Irving Langmuir made artificial membranes by adding phospholipids dissolved in benzene to water. a. benzene evaporates and lipid film covered the water with the hydrophilic beads of the phospholipid heads in the water 4. 1925: Gorter and Grendel reasn that cell membs. Must actually be phospholipid bilayers, two molecules thick a. measure memb. Content of RBS and realize there is enough lipid to cover cell with two layers 5. 1935: Hugh Davson and James Danielli propose sandwich model- phospholipid bilayer between two layers of globular protein a. supported by early microscope images b. accepted by 1960s but by later part of the decade, realized problems with model c. proteins over the lipid layer 6. 1972: Singer and Nicolson come up with a revised membrane model that places proteins in compatible location with their amphipatic nature a. propose that memb proteins are dispersed and inserted in the phopholipid layer, with their hyydrophilic region exposed to water b. fluid mosaic model’ c. freeze-fracture process provides proof - delaminate memb. Along middle of bilyaer splitting memb into outer and inner surfaces - halves viewed thru microscope, interior of bilayer appears to be cobblestned, with proten particles interspersed in matrix - proteins clearly penetrate hydrophobic interior or memb. B. Fluid mosaic Membrane Model 1. problems with the Davson-Danielli model a. generalization that all embranes of the cell are identical - don’t all look alike under microscope - mitochondrial membs. have different percentages of of components
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- mitochondrial membs. have different functions, and vary in chemical composition and structure b. placements of proteins - have hydrophobic and hydrophilic regions- are amphipathic like phospholipids - if proteins were on outside of membs, hydrophobic parts would be in an aqueous environment 1. fluidity a. memb. Held together by hydrophobic interaction, much weaker then covalent bonds b. lipids and some proteins can randomly drift about in memb c. reamins fluid as temp decreases d. at critical temp, phospholipids settle into a closely packed arrangement and memb solidifies - reamins fluid if phospholipids have unsaturated kinky hydrocarbons tails, which cannot pack together as closely e. steroid- cholesterol reduces memb fluidity by reducing phospholipid mvmnt at moderate temps, but also hinders solidification at low temps 2. mosaic or structure and function a. many diff proteins embedded in bilayer
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bio- chap 8 - I Membrane Structure A History ans Components...

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