Comparing all of the phospholipid fatty acid tail compositions listed below, which would generate a lipid bilayer with the most fluidity

Comparing all of the phospholipid fatty acid tail compositions listed below, which would generate a lipid bilayer with the most fluidity?

A Tails with 22 carbon atoms; both containing fully saturated hydrocarbon bonds.

B Tails with 22 carbon atoms; with one of the tails containing an unsaturated hydrocarbon bond.

C Tails with 18 carbon atoms; with one of the tails containing an unsaturated hydrocarbon bond.

D Tails with 20 carbon atoms; both containing fully saturated hydrocarbon bonds.

E Tails with 18 carbon atoms; both containing fully saturated hydrocarbon bonds.

The correct answer and explanation is:

The correct answer is C: Tails with 18 carbon atoms; with one of the tails containing an unsaturated hydrocarbon bond.

Explanation:

The fluidity of a lipid bilayer is significantly influenced by the composition of the phospholipid tails. Key factors that impact membrane fluidity include the length of the fatty acid chains and the presence of double bonds, which affect the packing of the lipid molecules within the bilayer.

1. Fatty Acid Chain Length:
   Shorter fatty acid chains generally increase membrane fluidity. Longer chains tend to be more rigid because they have more van der Waals interactions, making them less mobile. In this comparison, shorter chains (18 carbon atoms) would likely result in a more fluid bilayer than longer chains (22 carbon atoms or 20 carbon atoms), as shorter chains cannot pack as tightly as longer ones, thereby promoting more fluidity.
2. Saturation:
   Saturated fatty acids have only single bonds between carbon atoms, allowing them to pack more tightly together. This tight packing reduces the overall fluidity of the bilayer. On the other hand, unsaturated fatty acids contain at least one double bond, which introduces a kink in the tail. This kink prevents the fatty acid chains from packing tightly, thereby increasing the fluidity of the membrane. A bilayer with unsaturated fatty acids will be more fluid than one composed only of saturated fatty acids.

Now, let’s break down the options:

• Option A: Tails with 22 carbon atoms and fully saturated bonds would result in a relatively stiff, rigid bilayer, as the long and saturated tails would pack closely together, reducing fluidity.
• Option B: Tails with 22 carbon atoms, with one unsaturated bond, would be slightly more fluid than option A, but the long chain length still contributes to a relatively rigid structure.
• Option C: Tails with 18 carbon atoms, with one unsaturated bond, would generate the most fluid bilayer. The shorter 18-carbon tails reduce the packing density, and the presence of the unsaturated bond introduces a kink that prevents tight packing, both of which enhance fluidity.
• Option D: Tails with 20 carbon atoms and fully saturated bonds would be somewhat rigid, with longer chains promoting tight packing.
• Option E: Tails with 18 carbon atoms and fully saturated bonds would have more fluidity than options with longer saturated tails but would still be less fluid than option C because the absence of unsaturation prevents the chains from introducing the necessary kinks.

Thus, the combination of shorter chains and at least one unsaturated bond (option C) leads to the highest fluidity in the lipid bilayer.