The process by which solutes move down their concentration gradient without energy input is called

• A. Facilitated diffusion
• B. Active transport
• C. Endocytosis
• D. Simple diffusion

Answer: (D)

Moving solutes down their concentration gradient without energy input is simple diffusion. This is a passive process where particles spread from areas of higher concentration to lower concentration by passing directly through the lipid bilayer. Small nonpolar or lipid-soluble molecules, such as oxygen and carbon dioxide, cross this way without needing transport proteins or ATP. The other mechanisms involve energy or proteins: facilitated diffusion also moves substances down their gradient but uses a membrane channel or carrier; active transport requires ATP to push substances against the gradient; and endocytosis uses membrane vesicles to bring material into the cell, which is energy-dependent. Simple diffusion is the direct, no-energy path down the gradient.