Key Points The membrane is fluid but also fairly rigid and can burst if penetrated or if a cell takes in too much water. The mosaic nature of the plasma membrane allows a very fine needle to easily penetrate it without causing it to burst and allows it to self-seal when the needle is extracted. If saturated fatty acids are compressed by decreasing temperatures, they press in on each other, making a dense and fairly rigid membrane.
The ratio of saturated and unsaturated fatty acids determines the fluidity in the membrane at cold temperatures. Cholesterol functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity. Key Terms phospholipid : Any lipid consisting of a diglyceride combined with a phosphate group and a simple organic molecule such as choline or ethanolamine; they are important constituents of biological membranes fluidity : A measure of the extent to which something is fluid.
The reciprocal of its viscosity. Membrane Fluidity There are multiple factors that lead to membrane fluidity. Carbohydrates attached to lipids glycolipids and to proteins glycoproteins extend from the outward-facing surface of the membrane. The rate of transmembrane movement of cholesterol in the human erythrocyte. J Biol Chem. Lingwood D, Simons K. Lipid rafts as a membrane-organizing principle. Marsh D. CRC handbook of lipid bilayers. McElhaney RN. The relationship between membrane fluidity and phase state and the ability of bacteria and mycoplasmas to grow and survive at different temperatures.
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