Movement of substances from high concentration to low concentration is called what?

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

Answer: (A)

Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration, driven by the random motion of molecules and not requiring energy. It continues until the concentrations are equal on both sides. This sets it apart from other processes: osmosis is specifically the diffusion of water across a semi‑permeable membrane; filtration moves substances through a membrane due to a pressure difference; active transport uses energy to move substances against their concentration gradient. In practical terms, diffusion explains how small molecules spread down their concentration gradient without energy input.