Which statement best describes diffusion?

• A. Mass movement with water flow
• B. Active transport requiring energy
• C. Diffusion down the concentration gradient
• D. Gaseous exchange with the atmosphere

Answer: (C)

Diffusion is the passive spread of particles from areas of higher concentration to areas of lower concentration, driven by the random motion of molecules and not requiring energy. This is exactly what the statement captures when it says diffusion occurs down the concentration gradient. Because it moves along the gradient, diffusion happens spontaneously without energy input.

That’s different from bulk flow, which is mass movement with water, or from active transport, which requires energy to move substances against their gradient. Gaseous exchange with the atmosphere describes a situation diffusion helps with, but it’s describing the outcome rather than the mechanism itself. In soils or living tissues, diffusion drives nutrient and gas movement along concentration differences, though cells can also use energy-driven processes to move substances against those differences.