Membrane Fluidity from Lipid Saturation

Steps:

• Identify factors affecting membrane fluidity: temperature, cholesterol, and fatty acid saturation in phospholipids.
• Recall that saturated fatty acids pack tightly due to straight chains, reducing fluidity.
• Note that unsaturated fatty acids have double bonds creating kinks, loosening packing and increasing fluidity.
• Compare options: select the one with unsaturation for maximum fluidity contribution.

Why D is correct:

• Unsaturated fatty acids (D) introduce kinks from cis double bonds, disrupting tight packing and enhancing fluidity per the fluid mosaic model.

Why the others are wrong:

• A (saturated fatty acid): Straight chains allow dense packing, decreasing fluidity.
• B (cholesterol): Stabilizes membrane by filling gaps but reduces fluidity at physiological temperatures.
• C (trans fatty acid): Mimics saturation with straight chains, promoting tight packing and lower fluidity.

Final answer: D