What is diffusion?

• A. Movement from an area of greater concentration to an area of less concentration until equilibrium is reached
• B. Movement from low to high concentration
• C. Movement of water only
• D. Does not require a gradient

Answer: (A)

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by a concentration gradient, until equilibrium is reached. This is a passive process that does not require cellular energy. It applies to many substances—gases, ions, and solutes in a liquid—not just water. Osmosis is related but specifically refers to the diffusion of water across a selectively permeable membrane. The other statements aren’t correct because moving from low to high concentration would require energy, diffusion isn’t limited to water, and diffusion relies on a gradient; without one, there is no net directional movement (even though molecules may still move randomly).