Which term describes the movement of solutes through a semipermeable membrane from an area of higher concentration to an area of lower concentration until equilibrium is reached?

• A. Osmosis
• B. Active Transport
• C. Filtration
• D. Diffusion

Answer: (D)

Diffusion is the passive movement of solutes down their concentration gradient across a membrane, from areas of higher concentration to areas of lower concentration, until the solution becomes uniform. This process relies on random molecular motion and does not require cellular energy. The semipermeable membrane must allow the solute to pass; as the concentration equalizes on both sides, net movement stops. This differs from osmosis, which specifically involves water moving across a membrane; it also differs from filtration, which is driven by pressure rather than a concentration gradient, and from active transport, which requires energy to move substances against their gradient.