Cholesterol structure and amphipathic character Why is cholesterol considered amphipathic (amphiphilic) within eukaryotic membranes?

Introduction / Context:
Membrane lipids are amphipathic, possessing both hydrophobic and hydrophilic regions. Cholesterol, a major eukaryotic sterol, exemplifies this property and influences membrane fluidity and order.

Given Data / Assumptions:

• Cholesterol consists of a rigid tetracyclic steroid nucleus, a short hydrocarbon tail, and a single hydroxyl group at C-3.
• Membrane amphipathicity requires a distinct polar head and nonpolar body.

Concept / Approach:
The only polar moiety on cholesterol is its hydroxyl (–OH). This group hydrogen-bonds near the phospholipid headgroups, while the bulky fused ring system and isooctyl tail insert into the hydrophobic core. This duality makes cholesterol amphipathic and enables it to modulate membrane packing and phase behavior.

Step-by-Step Solution:
Identify the polar portion: the single hydroxyl group.Identify the nonpolar portion: the four-ring hydrophobic nucleus plus the hydrocarbon tail.Relate to membranes: OH aligns with aqueous interface; hydrophobic body aligns with acyl chains.Hence, amphipathicity arises from the –OH attached to an otherwise hydrocarbon framework.

Verification / Alternative check:
Experimental positioning in bilayers shows the hydroxyl near the phosphate region, consistent with hydrogen bonding to lipid headgroups.

Why Other Options Are Wrong:

• No polar group (options b, d): Without the –OH, cholesterol would not be amphipathic.
• Methyl groups (option c): Methyl substituents are nonpolar.
• Permanent positive charge: Cholesterol is uncharged at physiological pH.

Common Pitfalls:
Assuming ring count itself confers polarity; polarity is due to functional groups.

Final Answer:
Because a single hydroxyl group is attached to an otherwise hydrocarbon (steroid ring + tail) structure.