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Unformatted text preview: Chapter 7 – membrane structure and function Plasma membrane is the edge of life • I t is 8nm thick, and controls traffic into and out of the cell • Exhibits selective permeability – allows some substances to cross it more easily than others • Fundamental to life – ability of cell to discriminate in its chemical exchanges Cellular membranes are fluid mosaics of lipids and proteins • Made up of phospholipids – amphipathetic molecule o Has both a hydrophilic region and a hydrophobic region • Fluid mosaic model – membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer • Gorter and Grendel – came up with the phospholipid bilayer model o Could exist as a stable boundary • Surface of a membrane adheres less strongly to wter than does the surface of a biological membrane o Davsona nd Danielli suggested that this could be accounted for if the membrane was coated with hydrophilic proteins o Sandwich model – protein sandwhiching bilayer o But proteins were amphopathetic so couldn’t be in water • Singer and Nicolson proposed that membrane proteins are disperes individually inserted into the phospholipid bilayer with their hydrophilic regions protruding o Used a method called freeze-fracture – splits a membrane along the middle of the phospholibid bilayer o Shows the interior proteins Fluidity of Membranes • Held together by hydrophobic interactions which are much weaker than covalentbonds • Most of the lipids and proteins canshift laterally • Proteins are larger than lipids and move slower • Some membrane proteins seem to be held immobile in their attachment to the cytoskeleton • Unsaturated hydrocarbontails cannot pack together as tightly as saturated ones so it makes the membrane more fluid • Steroid cholesterol which is wedged between phospholipid molecules has different effects on membrane fluidity o High tmps – cholestorol makes membrane less fluid by restraining phospholipid movement o Hinders close packing of temperatures – lowers the temp required for membrane to solidify • Cholesterol = temperature buffer for membranes o Membranes must be fluid to work o When solid – enzymes become inactive Membrane Proteins and their functions...
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This note was uploaded on 10/07/2010 for the course HELLO IDK taught by Professor Armand during the Spring '10 term at Barber-Scotia.
- Spring '10