Quest_Osmosis - Selective Permeability (Membrane) - Allows...

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Selective Permeability (Membrane) - Allows some substances to cross more easily than others - The ability of the cell to discriminate in its chemical exchanges with its environment is impt to life - Made of lipids and proteins *Found out by analyzing membranes of red blood cells Phospholipids - Amphipathic molecule *Both a hydrophobic and hydrophilic region - Most proteins within the membrane are amphipathic Fluid Mosaic Model - The membrane is a fluid structure with a mosaic of various proteins embedded in or attached to a double layer of phospholipids 1925 - Molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing hydrophilic heads to water - Problem: protein location *Surface of a membrane consisting of a pure phospholipid bilayer adheres less strongly to water than the surface of a biological molecule 1935 - Sandwich model *Membrane coated on both sides with hydrophilic proteins *Phospholipid bilayer between two layers of proteins - Two problems with model: 1) Assumes all membranes are the same *Membranes with different functions differ in chemical composition and structure 2) Protein placement *Membrane proteins aren’t soluble in water *If they were layered on the surface of the membrane, the hydrophobic parts would be surrounded by water 1972 - Membrane proteins are dispersed, individually inserted into the phospholipid bilayer with their hydrophilic regions protruding
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Freeze Fracture - Method of preparing cells for electron microscopy -Splits membrane along the middle of the phospholipid bilayer Fluidity of Membrane - Held together by hydrophobic interactions - Lipids and some proteins can shift about laterally (in the plane of the membrane) *Adjacent phospholipids switch positions 10^7 times/sec Frye & Edidin - Some membrane proteins seem to move in a highly directly manner - Others are immobile b/c of their attachment to the cytoskeleton - Marked membrane proteins on mouse and human cells and fused then together, they mixed after an hour *Indicated that at least some membrane proteins move sideways within the plane of the membrane Temperature and Fluidity - As temp decreases phospholipids settle into a closely packed arrangement & membrane solidifies - Membrane stays fluid to a lower temp if it’s rich with phospholipids with
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Quest_Osmosis - Selective Permeability (Membrane) - Allows...

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