The movement of a solute from an area of high concentration to an area of low concentration is called

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

Answer: (A)

Movement from a region of high concentration to a region of low concentration is diffusion, the natural spread of molecules due to random motion. When small, nonpolar molecules cross the cell membrane directly through the lipid bilayer without any energy input or transport proteins, that is simple diffusion. This is passive transport, meaning it doesn't require cellular energy and proceeds down the concentration gradient toward equilibrium. Other mechanisms either use transport proteins (facilitated diffusion) or require energy to move against the gradient (active transport) or involve vesicle formation for uptake (endocytosis), so they don’t fit the description as directly as simple diffusion.