Question

Cholesterol is a plasticizer as

• A. It forms the bilayer and increases membrane fluidity at high temperature
• B. It is not rigid steroid ring system
• C. It does not interfere with the motions of the fatty acid side chain in other membrane lipids
• D. Inhibits the order in the fatty acid chains by fitting in between them

Hint:

Cholesterol Fluidity Buffer: Prevents membrane crystallization/gelation at low temp AND prevents excessive fluidity at high temp by fitting between acyl chains.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Cholesterol functions as a bidirectional membrane fluidity buffer (plasticizer) by intercalating between phospholipid acyl chains.

Step 2: Detailed Explanation:
Biophysics of cholesterol as a Membrane Fluidity Buffer (Plasticizer):
- The rigid, planar four-ring steroid nucleus of cholesterol intercalates between the parallel hydrocarbon fatty acid tails of phospholipids in the lipid bilayer.
- At Lower Temperatures: Cholesterol prevents the tightly ordered crystalline packing and crystallization of adjacent hydrocarbon chains, preventing gel phase formation and maintaining fluidity.
- At Higher Temperatures: Its rigid steroid core restricts excessive rotational wobble and chaotic motion of acyl chains, preventing excessive fluidity.
- Thus, by disrupting crystalline order and fitting in between acyl chains, it plasticizes the membrane.

Step 3: Final Answer:
Thus, cholesterol acts as a plasticizer by inhibiting the order in fatty acid chains by fitting between them, corresponding to option (D).