Which molecule is amphipathic and a common component of animal cell membranes?

• A. Cholesterol
• B. Glucose
• C. Sucrose
• D. DNA

Answer: (A)

Amphipathic molecules have both a hydrophilic (water-loving) part and a hydrophobic (water-fearing) part, which lets them sit comfortably at the lipid–water interface in membranes. Cholesterol fits this pattern: its hydrophobic four-ring steroid structure interacts with the fatty acid tails of the phospholipids, while a single hydroxyl group provides a small polar region that can associate near the phospholipid head groups. In animal cell membranes, cholesterol inserts between phospholipids, with the hydrophobic part embedded in the inner part of the bilayer and the hydroxyl near the membrane surface. This arrangement helps regulate membrane fluidity and stability, preventing the membrane from becoming too rigid or too permeable under different temperatures. Glucose and sucrose are polar sugars, not amphipathic in a way that would integrate into the bilayer similarly, and DNA is a charged polymer lacking a hydrophobic region to anchor within the membrane. So cholesterol is the amphipathic molecule commonly found in animal membranes.