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Chapter 7 � membrane structure and function

Chapter 7 � membrane structure and function - C...

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Chapter 7 – membrane structure and function Plasma membrane is the edge of life It 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
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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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