What molecular feature dictates the fluidity of a lipid bilayer?

• A. Length of the fatty acid chains
• B. Degree of saturation of fatty acids
• C. Presence of cholesterol
• D. Type of phospholipids used

Answer: (B)

The degree of saturation of fatty acids significantly influences the fluidity of a lipid bilayer. Fatty acids can be either saturated, having no double bonds between the carbon atoms in their hydrocarbon chains, or unsaturated, containing one or more double bonds. Unsaturated fatty acids introduce kinks in the hydrocarbon chains, which prevent the molecules from packing closely together. This increased spacing among the lipid molecules enhances the fluidity of the membrane.

On the other hand, saturated fatty acids, which can pack tightly together due to their straight chains, tend to result in a more rigid and less fluid bilayer. Therefore, highly unsaturated fatty acid chains contribute to a more fluid membrane compared to chains that are predominantly saturated.

While factors such as the length of fatty acid chains, cholesterol presence, and the type of phospholipids do play roles in determining membrane characteristics, it is the saturation level of the fatty acid chains that is most directly responsible for altering fluidity in a pronounced way. The interplay between these factors collectively contributes to the overall properties of biological membranes, but the degree of saturation is the primary determinant in fluidity changes.