How does cholesterol typically affect phospholipid bilayer properties at physiological temperatures?

Question

BlackTom AI · Accepted Answer

Cholesterol's rigid sterol ring and short polar hydroxyl group intercalate between phospholipid molecules, so it orders nearby acyl chains and fills gaps in the bilayer.

1) Relevant knowledge points and concepts
- Cholesterol structure: rigid planar steroid rings with a single polar OH and a short hydrocarbon tail.
- Membrane packing: phospholipid acyl chains have thermal motion that determines fluidity and permeability.
- Cholesterol effects: at physiological temperatures (e.g., ~37°C) cholesterol tends to decrease lipid mobility and reduce free volume between lipids; it also broadens and reduces the sharpness of the gel–liquid crystalline phase transition (buffers temperature sensitivity).

2) Step-by-step reasoning
- The OH group orients toward phospholipid headgroups; the sterol rings insert into the hydrophobic core.
- These rigid rings restrict rotation and lateral motion of neighboring fatty acid chains, so chain order increases.
- Increased order reduces lateral diffusion and the ability of small solutes to slip between lipids, thus lowering permeability.
- By disrupting cooperative transitions between ordered and disordered states, cholesterol makes membranes less responsive to small temperature changes (temperature-buffering).

3) Intermediate logical steps (no numeric calculation required)
- Insert cholesterol → decreases acyl chain motional freedom → lower membrane fluidity.
- Less free volume + tighter packing → reduced permeability to water and small solutes.
- Disruption of phase transition cooperativity → decreased sensitivity to temperature.

4) Why the correct answer is correct
The choice states that cholesterol "decreases membrane permeability and fluidity, making membranes less sensitive to temperature changes." This matches molecular evidence: cholesterol orders acyl chains (reducing fluidity), fills voids (reducing permeability), and broadens phase transitions (temperature buffering).

5) Why other options are incorrect
- “Destabilises membranes and promotes immediate lysis”: incorrect — cholesterol stabilizes bilayers rather than causing lysis.
- “Functions as the primary energy source for membrane proteins”: incorrect — cholesterol is structural, not an energy substrate.
- “Irreversibly cross-links phospholipids into rigid crystals”: incorrect — interactions are noncovalent and reversible; membranes remain fluid, not crystalline.
- “Creates large aqueous pores that increase permeability dramatically”: incorrect — cholesterol decreases, not increases, permeability.

6) Conclusion
Therefore, the correct description is: It decreases membrane permeability and fluidity, making membranes less sensitive to temperature changes — which matches the provided answer.

How does cholesterol typically affect phospholipid bilayer properties at physiological temperatures?单项选择题

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