What is osmosis?

• A. Diffusion of solutes across a semipermeable membrane.
• B. Active transport of water across a membrane.
• C. Facilitated diffusion of water across a membrane.
• D. Diffusion of water across a selectively permeable membrane.

Answer: (D)

Osmosis is the movement of water across a membrane that is permeable to water but selectively permeable to solutes. Water tends to move from the side with a lower concentration of dissolved substances to the side with a higher concentration, so that the overall solute concentration becomes more balanced. This is a passive process, meaning it does not require cellular energy, and water can cross the membrane by simple movement through the lipid bilayer or via water channels called aquaporins. This description matches the idea of diffusion of water across a selectively permeable membrane. The other ideas mix in solute diffusion, active transport, or a form of facilitated diffusion that isn’t the defining feature of osmosis.