Membrane
Fluidity
Biological membranes are fluid in nature. For example, when individual cells with different surface protein markers are fused, the initially separated proteins rapidly mix on the newly formed hybrid (Figure 10.11). This phenomenon is known as lateral diffusion, because molecules move laterally within the plane of the membrane. By contrast, in the much less frequent transverse diffusion, a molecule moves from one side of the lipid bilayer to the other.
The fluidity of a membrane is a function of its lipid composition and temerature. For example, lower temperatures tend to decrease membrane fluidity because the molecules of the lipid bilayer tend to form regular, more rigid crystalline structures. Higher temperatures favor less regular, more gel-like structures. The temperature at which a membrane converts from crystalline to gel-like structures is referred to as the transition temperature (Figure 10.12).
The transition temperature itself depends on the composition of the lipids in the membrane. Membrane lipids with shorter fatty acids or unsaturated fatty acids have lower transition temperatures. In addition, the composition of the polar head group can have a drastic effect. For example, an ethanolamine head group raises the transition temperature compared to a choline head group. Organisms have membrane compositions consistent with transition temperatures somewhat below their lowest body temperature.
Cholesterol has an interesting effect on membrane fluidity. As seen in Figure 10.12b, cholesterol does not change the transition temperature of a membrane, but rather broadens the range of the transition considerably. It has been hypothesized that this broadening occurs because cholesterol can both stiffen the membrane above the transition temperature and inhibit regularity in structure formation below the transition temperature. Thus, it blurs the distinction between the gel and fluid state.
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