Decreasing saturated fatty acids increases membrane fluidity

Steps:

• Membrane fluidity depends on how tightly phospholipids pack in the bilayer.
• Saturated fatty acids form straight chains without kinks, promoting tight packing and low fluidity.
• Unsaturated fatty acids have kinks from double bonds, spacing chains apart for higher fluidity.
• Decreasing saturated fatty acid proportion shifts to more unsaturated chains, loosening packing.

Why C is correct:

• Saturated fatty acids lack double bonds, forming straight chains that maximize van der Waals attractions and reduce fluidity; decreasing their proportion introduces kinks, disrupting packing and increasing fluidity per the fluid mosaic model.

Why the others are wrong:

• A: Short fatty acid chains limit interactions and increase fluidity; decreasing them promotes tighter packing, reducing fluidity.
• B: Distance between molecules indicates spacing; decreasing it causes closer packing, reducing fluidity.
• D: Long fatty acid chains extend interactions but secondary to saturation; decreasing them increases fluidity, though saturation primarily controls kinking and packing.

Final answer: C