What is the role of aquaporins?

• A. Enzymes that metabolize sugars
• B. Water channels that facilitate water transport
• C. Channels that move ions across membranes
• D. Receptors for signaling molecules

Answer: (B)

Aquaporins are specialized membrane proteins that form water-selective channels, allowing water to move rapidly across cell membranes in response to osmotic gradients. This facilitated water transport is crucial because water diffusion through the lipid bilayer is relatively slow, and organisms rely on these channels to quickly balance water inside and outside cells. In tissues like the kidney, aquaporins enable precise water reabsorption to concentrate urine, while in plants they help adjust cell turgor as environmental conditions change. While the classic aquaporins primarily transport water, some related proteins (aquaglyceroporins) can also allow small uncharged solutes like glycerol. They are not enzymes that metabolize sugars, not ion channels for charged particles, and not receptors for signaling molecules, which is why the role described here is specifically about moving water across membranes.