Role of cholesterol in eukaryotic membranes: Why is cholesterol essential for normal membrane function?

Introduction / Context:
Cholesterol is a major sterol in animal cell membranes. It modulates membrane physical properties, which in turn affect protein function, permeability, and signaling domains.

Given Data / Assumptions:

• Eukaryotic membranes contain varying amounts of cholesterol (high in animal cells).
• Question asks for the correct functional role.

Concept / Approach:
Cholesterol intercalates among phospholipid tails, reducing excessive fluidity at high temperatures and preventing tight packing and gel formation at low temperatures. Net effect: maintenance of appropriate membrane fluidity and decreased permeability to small solutes.

Step-by-Step Solution:
Consider physical role → fluidity buffer and ordering agent. Reject answers unrelated to membrane physics (arterial plaques, ATP source). Note cholesterol does not span bilayer; it sits within one leaflet depth influencing order. Select option stating “keeps membranes fluid”.

Verification / Alternative check:
Differential scanning calorimetry and diffusion studies show cholesterol broadens phase transitions and stabilizes liquid-ordered states.

Why Other Options Are Wrong:
(a) Pathological statement unrelated to function; (b) higher organisms do synthesize cholesterol; (d) not a transmembrane protein; (e) cholesterol is not an energy currency.

Common Pitfalls:
Assuming cholesterol only “stiffens” membranes; in fact, it buffers fluidity depending on temperature and composition.

Final Answer:
It keeps membranes fluid.