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Cholesterol and Fatty Acids Regulate Membrane Fluidity
The fluidity of the membrane is a function of the relative movement of the lipids. The relative movement is itself a property of the strength the intermolecular interactions holding the lipids together. The stronger these interactions are, the more rigid the membrane is. Suppose that we have a very rigid membrane; as you heat the membrane and increase the temperature, eventually a temperature will be reached (called the melting temperature) at which a phase transition will occur and the membrane will become fluid. This transition takes places because the increase in temperature gives the lipids and proteins a greater kinetic energy; this means that the intermolecular bonds cannot maintain the well-ordered structure of the rigid membrane. We see that the stronger these intermolecular bonds are, the more rigid the membrane is and the higher the melting temperature is. There are three factors that influence the strength of these bonds and therefore influence fluidity - (1) length of the fatty acids (2) degree of unsaturation of the fatty acids and (3) cholesterol. As you increase the length of the fatty acids in the membrane, there are more London dispersion forces and this increases the strength of the intermolecular bonds. Therefore, increasing the length will make the membrane more rigid (less fluid) and increase the melting temperature. Unsaturated fatty acids that contain cis double bonds will increase the fluidity of the membrane because they introduce kinds, or bends, in the well-ordered structure of the membrane. This decreases the overall strength of the intermolecular bonds and lowers the melting temperature. Lastly, what about cholesterol? Although cholesterol interferes with the well-packed nature of the fatty acid chains, it stimulates the formation of cholesterol-glycophospholipid complexes. These complexes in turn form larger complexes called lipid rafts. Lipid rafts are regions of the membrane that contains a densely-packed region of cholesterol and glycophospholipids. This decreases the overall motion of the membrane and makes it more rigid. In addition, it increases the ability of the membrane to resist phase transitions. This is very important in animal cells because it helps them maintain homeostasis.
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