Membrane Biochemistry—Physiological Role of Cholesterol In animal cell membranes, what is an important function of cholesterol under physiological temperatures?

Introduction / Context:
Cholesterol is a major sterol in animal plasma membranes. Its rigid ring system and small polar headgroup allow it to intercalate among phospholipids and tune physical properties of the bilayer. Recognizing its biophysical role helps explain membrane integrity, permeability, and protein function.

Given Data / Assumptions:

• Cholesterol inserts between phospholipid acyl chains.
• Membranes must remain within an optimal fluidity window for protein activity and transport.
• Physiological temperature range is considered.

Concept / Approach:
Cholesterol modulates membrane fluidity and order: it restrains phospholipid movement at higher temperatures (reducing excessive fluidity) and prevents tight packing at lower temperatures (inhibiting gel formation). Net effect: buffering membrane fluidity and decreasing permeability to small solutes.

Step-by-Step Solution:

Verification / Alternative check:
Experimental measurements (e.g., fluorescence anisotropy) show cholesterol-dependent changes in membrane order parameters and diffusion.

Why Other Options Are Wrong:

• Enhancing blood circulation: systemic physiology, not a direct membrane function.
• Preventing bile salt formation: bile acids are synthesized from cholesterol; cholesterol does not prevent their formation.
• Increasing electrical conductivity directly: cholesterol is not an ion channel.
• “None of these” is incorrect because a key function is known.

Common Pitfalls:
Confusing cholesterol’s role in membranes with its role as a synthetic precursor of bile acids and steroid hormones.

Final Answer:
Modulate fluidity