Which statement best contrasts diffusion and osmosis?

• A. Diffusion is the spontaneous movement of particles down a concentration gradient; osmosis is diffusion of water through a selectively permeable membrane
• B. Diffusion requires energy; osmosis does not
• C. Osmosis is active transport; diffusion is passive
• D. Both describe movement of water only

Answer: (A)

The main idea here is that diffusion is the spontaneous spread of particles from higher to lower concentration, while osmosis is a specific type of diffusion that involves water moving across a selectively permeable membrane. Diffusion applies to any solute, driven by Brownian motion, whereas osmosis specifically describes water moving to balance solute concentrations across a membrane (water moves toward higher solute concentration, lowering water potential on the side with more solute).

That distinction is why the statement is the best: it correctly describes diffusion as movement down a concentration gradient and identifies osmosis as the diffusion of water through a selectively permeable membrane. Other choices misstate energy requirements, or mislabel osmosis as active transport, or claim diffusion and osmosis only involve water movement.