bio2ch5 - Chapter Five Cellular Membranes 1 Membranes a...

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Chapter Five Cellular Membranes 1) Membranes a) Regulate traffic in and out of the cell b) Lipids i) Create an impermeable barrier to water-soluble molecules ii) Establish the physical integrity of the membrane iii) Lipids are like the water of a lake in which proteins “float” c) Proteins are embedded in the membrane d) Carbohydrates are attached to lipid or protein moecules i) On the outside of the plasma membrane 2) Fluid Mosaic Model 3) Lipids a) Make up the bulk of the membrane b) Mostly phospholipids c) Organized into a bilayer d) Hydrophilic regions face the outside of the cell and the cytoplasm e) Hydrophobic regions face each other f) Interior of the membrane is fluid i) Allows some molecules to move laterally within the plane of the membrane ii) Lipid molecules can move laterally as well iii) A phospholipid molecule rarely flip-flops in the membrane (1) This requires help from special proteins 4) Membranes a) All biological membranes have similar structures b) Some have different lipid and protein composition c) Some have 25% cholesterol i) Important in membrane integrity ii) Can either increase or decrease membrane fluidity 5) Effects of Cholesterol depends on… a) The fatty acid composition of other lipids b) Membrane fluidity decreases with decreases in temperature i) Decline in membrane function ii) Organisms change the lipid composition of membranes under cold conditions 6) Membrane Proteins a) Asymmetrical distribution b) Usually 1 protein for every 25 phospholipids i) Mitochondrial membrane – 1 protein for every 15 phospholipids ii) In myelin – 1 protein for every 70 phospholipids c) many proteins have both hydrophilic and hydrophobic regions i) form noncovalent interactions with lipids in the membrane d) Integral membrane proteins i) Have long stretches of hydrophobic amino acids ii) Completely crosses the bilayer iii) Form an alpha-helical region that interacts only with the hydrophobic core
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iv) Hydrophilic ends extend into the aqueous environment 7) Transmembrane Proteins a) Extend from the membrane on both sides b) Have a specific orientation i) Outside of the cell = shaped like a receptor ii) Inside portion changes shape in response to signal binding 8) Peripheral Proteins a) Lack hydrophobic regions b) Interact with exposed regions of integral proteins or phospholipids 9) The Membrane is Fluid a) Many proteins are free to move around in the membrane b) Some proteins are anchored to a specific region of the membrane i) Cytoskeleton
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bio2ch5 - Chapter Five Cellular Membranes 1 Membranes a...

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