which of the following statements best describes how unsaturated fatty acids increase membrane fluidity at lower temperatures?

Unsaturated fatty acids increase membrane fluidity at lower temperatures because their kinked hydrocarbon chains (due to cis double bonds) prevent tight packing of phospholipids, keeping the bilayer more disordered and thus more fluid.

Core idea

• Unsaturated fatty acids contain one or more cis double bonds that introduce bends or kinks in the fatty acid tails.

• These kinks stop phospholipid tails from aligning closely, which weakens van der Waals interactions between them and lowers the temperature at which the membrane would otherwise solidify.

What this means for membrane fluidity

• At low temperatures, membranes tend to become more ordered and gel-like; adding more unsaturated fatty acids counters this by maintaining a looser, more fluid arrangement of lipids.

• In exam-style wording, the best description is:

“The cis double bonds in unsaturated fatty acid tails introduce kinks that reduce tight packing of phospholipids, lowering the melting (transition) temperature and increasing membrane fluidity at lower temperatures.”

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