Which characteristic of a phospholipid contributes to the fluidity of the membrane? a its head b cholesterol c a saturated fatty acid tail d double bonds in the fatty acid tail?

The characteristic of a phospholipid that contributes to the fluidity of the membrane is D. double bonds in the fatty acid tail.

Phospholipid simply means the molecules that have hydrophilic phosphate heads as well as the hydrophobic lipid tails.

Phospholipid is vital for the regulation of cellular processes. The fluidity of the membrane for a phospholipid is due to the fact that there are double bonds in the fatty acid tail.

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The answer is D. The double binds in the phospholipids tail ensure that the chains are not well compacted ( because the van der Waals interactions with neighboring hydrophobic chains are not well matched due to kinks in the chains.) hence the membrane is not hard and is fluid.  Cholesterol also intercalated in between the chains in most cell membranes and additionally prevents them from clustering together and stiffening when the temperatures are low.