Which cellular component forms water channels in the membrane?

• A. Aquaporins
• B. Cholesterol
• C. Phospholipids
• D. Proteins

Answer: (A)

Water moves across membranes most efficiently through specialized pore-forming proteins. Aquaporins are integral membrane proteins that assemble to create a narrow pore, allowing water to pass rapidly in response to osmotic differences while excluding ions and other solutes. In contrast, cholesterol sits among the lipid bilayer and mainly affects membrane fluidity and rigidity; phospholipids form the bilayer barrier and do not provide a selective channel; proteins is too general a category, since not all proteins form water channels, whereas aquaporins specifically do. Therefore, aquaporins are the best answer because they are the exact molecular structures that create water channels in membranes.