Which statement best describes diffusion?

• A. External energy input
• B. Random motion of particles
• C. Active transport using ATP
• D. Osmosis

Answer: (B)

Diffusion is the passive spread of substances due to the constant, random motion of molecules. This random motion causes particles to move from regions of higher concentration to regions of lower concentration, and the net movement continues until concentrations are evened out. No external energy input is required because the process relies on inherent molecular motion, not on cellular pumps or metabolic energy. Osmosis is a specific type of diffusion—the movement of water across a semipermeable membrane—so while related, it’s not the general description of diffusion itself. Active transport, on the other hand, uses energy to move substances against their gradient, which is not diffusion. Therefore, the best description is that diffusion arises from the random motion of particles driving them down their concentration gradient.