Fluid Mosaic Model Essay

638 WordsDec 2, 20133 Pages
The Fluid-Mosaic Model of a Plasma Membrane The fluid mosaic model of S.J.Singer and G.L. Nicolson (1972), which replaced the earlier model of Davson and Danielli, describes how biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily. Although the lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by themselves, the plasma membrane also contains a large quantity of proteins, which provide more structure. According to the model, proteins are individuals embedded in the phospholipid bilayer, rather than forming a solid coat spread upon the surface. Hydrophilic portions of both proteins and phospholipids are maximally exposed to water resulting in a stable membrane structure. Hydrophobic portions of the proteins and phospholipids are in the nonaqueous environment inside the bilayer. The membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids. Evidence from freeze fracture techniques have confirmed that proteins are embedded in the membrane. Different proteins are embedded and dispersed in the phospholipid bilayer. Integral proteins are transmembrane proteins with hydrophobic interiors of the membranes. Peripheral proteins are attached to the membranes surface. The membrane is held together by hydrophobic interactions, which are weak attractions. Most membranes lipids and some proteins can drift laterally within the membrane. Unsaturated hydrocarbon tails enhance membrane fluidity, because kinks at the carbon to carbon double bond hinder close packing of the phospholipids. Membranes solidify if the temperature decreases to a critical point. Critical temperature is lower in membranes with a greater concentration of unsaturated phospholipids. Cholesterol modulates membrane fluidity by making the membrane less fluid at

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