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BCHE 395-16 2014 Membranes and Transport I

O o p o polar head cholesterol ho o hydrophobic tail

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Unformatted text preview: d by a complex of lipids in the bilayer, studded by a variety of proteins and other “decoraFons” •  Very dynamic structure with both lipids and proteins constantly in moFon, including rapid, lateral diffusion •  Each type of membrane has characterisFc lipids and proteins Differences within a cell Differences between organisms Membrane ComposiFon Membrane ComposiFon •  Cholesterol content is an important determinant of lipid state •  Rigid planar state of cholesterol reduces freedom of mo8on of neighboring phospholipid tails •  So cholesterol REDUCES fluidity of the bilayer Phospholipid O O P O Polar "head" Cholesterol HO O Hydrophobic "tail" Cholesterol •  Small amounts of cholesterol are important to maintain fluidity •  Excess cholesterol elevates the transiFon temp and “freezes” the membrane Membrane ProperFes •  Lipids and proteins are in constant moFon (lateral) •  Transversing the lipid bilayer is very slow/ RARE (also known as “flip- flop”) Membrane Diffusion: Flippases •  Special enzymes—flippases—catalyze transverse diffusion •  Some flippases use energy of ATP to move lipids against the concentraFon gradient Differences in fluidity between leaflets? You Betcha! Janus Membrane Asymmetry •  Biological membranes have 2 disFnct “faces:” an inner and an outer leaflet •  Ex. Erythrocyte Membrane Asymmetry Freeze- ƒracture Electron Microscopy •  So how did they determine the composiFon of the inner and outer leaflets? •  A flash frozen membrane is sha\ered along the leaflet plane of the bilayer •  Pealed leaf reveals internal structure of the membrane bioweb.wku.edu Freeze- Fracture umassmed.edu Membrane Asymmetry Think about which amino...
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