Osmosis is best described as which process?

• A. A process in which water diffuses through a selectively permeable membrane
• B. The diffusion of solutes across a membrane along their concentration gradient
• C. The movement of water against its concentration gradient using energy
• D. The evaporation of water from a surface

Answer: (A)

Osmosis is water moving across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration, a passive process that requires no energy. This movement happens because water moves to dilute the more concentrated solution, and the membrane allows water to pass while restricting many solutes. Aquaporin channels in membranes often facilitate this flow.

That’s why the description of water diffusing through a selectively permeable membrane best captures osmosis. The other ideas describe diffusion of solutes (not water), movement of water against its gradient with energy (active transport), or evaporation, none of which define osmosis.