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The enhancement of fluidity in a lipid bilayer is largely influenced by the degree of unsaturation in the fatty acid tails of the phospholipids that make up the membrane. Unsaturated fatty acids contain one or more double bonds that create kinks in their structure, preventing the fatty acids from packing closely together. This increased spacing between the lipid molecules allows for greater movement within the bilayer, thereby enhancing fluidity.

In contrast, saturated fatty acid tails, which contain no double bonds, can pack tightly together, resulting in a more rigid structure and decreased fluidity. While cholesterol can modulate membrane fluidity depending on its concentration and the existing environment, it typically serves to stabilize the membrane and can reduce fluidity at certain concentrations. Higher temperatures can indeed increase fluidity, but they are not the only factor contributing to this property. Therefore, the primary and direct enhancer of membrane fluidity in this context is the presence of increased unsaturation in the fatty acid tails.