Which statement best describes diffusion?

• A. Movement of solutes from high to low concentration down a gradient
• B. Movement of water across a membrane
• C. Movement of ions against their gradient
• D. Movement of water from low to high concentration

Answer: (A)

Diffusion is the spreading of particles from an area of higher concentration to an area of lower concentration because of their random motion. It is a passive process, meaning no energy is required. The statement that best describes diffusion is the one that says solutes move down a concentration gradient, from high to low concentration. This captures the essence of diffusion as particles naturally spread out until evenly mixed.

Other descriptions refer to different processes: movement of water across a membrane is osmosis, not diffusion; movement of ions against their gradient is active transport, which uses energy; and water moving from low to high concentration isn’t how diffusion is described.