Many membrane processes, such as transport or signal transduction, depend on the fluidity of the membrane lipids, which is determined by the properties of fatty acid chains. Recall that the melting point of individual fatty acids depends on their length and the number of cis double bonds. These same chemical properties affect the fluidity of the membranes of which the fatty acids are a component. The fatty acid chains in membrane bilayers may be arranged in an ordered, rigid state or in a relatively disordered, fluid state. The transition from the rigid to the fluid state takes place rather abruptly as the temperature is raised above T_m, the melting temperature (Figure 12.5). This transition temperature depends on the length of the fatty acid chains and on their degree of unsaturation. Long saturated fatty acids interact more strongly because of the increased number of van der Waals interactions than do short ones and thus favor the rigid state (Figure 12.6). On the other hand, a cis double bond produces a bend in the hydrocarbon chain. This bend interferes with a highly ordered packing of fatty acid chains, and so T_m is lowered.

Bacteria regulate the fluidity of their membranes by varying the number of double bonds and the length of their fatty acid chains. In animals, cholesterol is the key modulator of membrane fluidity. Cholesterol contains a bulky steroid nucleus with a hydroxyl group at one end and a flexible hydrocarbon tail at the other end. The molecule inserts into bilayers with its long axis perpendicular to the plane of the membrane. Cholesterol’s hydroxyl group forms a hydrogen bond with a carbonyl oxygen atom of a phospholipid head group, whereas its hydrocarbon tail is located in the nonpolar core of the bilayer. The different shape of cholesterol compared with that of phospholipids disrupts the regular interactions between fatty acid chains and helps maintain membrane fluidity (Figure 12.7).

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In addition to its nonspecific effects on membrane fluidity, cholesterol can form specific complexes with saturated fatty acid components of lipids and specific proteins. These complexes concentrate within small (10–200 nm) and highly dynamic regions within membranes. The resulting structures are often referred to as lipid rafts. One result of these interactions is the moderation of membrane fluidity, making membranes less fluid but at the same time less subject to phase transitions. Although their small size and dynamic nature have made them verydifficult to study, lipid rafts play a role in concentrating proteins required for signal-transduction pathways, regulate membrane curvature and budding, and facilitate the interaction between the extracellular matrix and the cytoskeleton.