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Chapter 7 - Chapter 7 Membrane Structure and Function I...

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Chapter 7 – Membrane Structure and Function I. General Information A. Plasma membrane defines and limits the cell B. Controls entrance and exit from the cell C. Selective permeability – allows certain substances in while excluding others II. Cellular Membranes are Fluid Mosaics of Lipids and Proteins A. Membranes are predominantly phospholipids B. Both membrane phospholipids and proteins are amphipathic (the molecules have both hydrophilic and hydrophobic regions on the same molecule) C. Carbohydrates are usually limited to the outer membrane D. Fluid mosaic model – a fluid matrix of phospholipids with a mosaic of proteins embedded in it E. Made of proteins and lipids F. Models 1. Freeze fracture 2. Sandwich model – supported originally by electron microscopy but had problems explaining permeability 3. False assumption that all membranes were identical 4. Fluid mosaic model was found to be consistent with the known physiological characteristics of living membranes G. Fluidity of Membranes 1. Lateral movement (10^7), Flip-flop (once per month)-movement of phospholipids 2. Membranes are held together by hydrophobic interactions 3. Molecules float about in the membrane 4. Membrane molecules are in constant rapid motion 5. Some proteins are structurally controlled in their mobility. Some cannot move at all. They are controlled by the cytoskeleton 6. Membranes with unsaturated hydrocarbons (fatty acids) resist cooling more than those with saturated hydrocarbons 7. Membranes are usually about as fluid as salad oil
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8. Cholesterol helps stabilize membranes by impeding the movement of phospholipids. It is called a “temperature buffer” 9. Cholesterol like antifreeze in membranes 10.A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer 11.Proteins determine most of the membrane’s specific fxns 12.Solidification of a membrane usually inactivates its
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