Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologyhardmcq1 pt

Membranes of fish living in icy water contain a higher share of unsaturated fatty acid tails than membranes of fish in warm water. How do the unsaturated tails keep the membrane working in the cold?

A.Cis double bonds put kinks in the tails, so the tails pack loosely and the bilayer stays fluid
B.The extra double bonds store energy that the membrane burns to heat itself
C.The kinked tails expel water from the bilayer and raise its surface tension
D.The unsaturated tails bind cholesterol so tightly that fluidity is locked in place

Explanation

Core Concept

Membrane function depends on phospholipids staying mobile enough to let proteins move and small molecules pass. Unsaturated tails contain cis double bonds that bend the hydrocarbon chain, so neighboring tails cannot align into the tight, orderly array that solidifies into a gel. Loose packing preserves fluidity at low temperature, which is why cold-adapted organisms raise the unsaturated share of their membrane lipids. Energy storage, water exclusion, and cholesterol binding do not explain this cold adaptation.

Correct Answer

ACis double bonds put kinks in the tails, so the tails pack loosely and the bilayer stays fluid

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