Which statement best defines osmosis?

• A. Diffusion of solutes across a selectively permeable membrane from high solute concentration to low.
• B. Diffusion of water across a selectively permeable membrane from an area of higher water potential to a lower water potential.
• C. Active transport of water using cellular energy.
• D. Osmosis describes the movement of any molecule through a membrane.

Answer: (B)

Osmosis is the diffusion of water across a selectively permeable membrane from an area of higher water potential to a lower water potential. Water potential measures water’s tendency to move; adding solutes lowers this potential, so water moves toward solutions with lower potential through a membrane that allows water to pass. It’s a passive process, requiring no energy. The other statements describe diffusion of solutes, active transport, or movement of any molecule—none of which define osmosis as precisely as the water-moving, potential-driven flow described here.