Diffusion is described as

• A. It requires energy input to move molecules from high to low concentration
• B. It moves ions against a membrane without channels
• C. It spreads from high to low concentration with energy input
• D. It spreads from high to low concentration without energy input

Answer: (D)

Diffusion is a passive process driven by a concentration gradient: particles move from regions of higher concentration to regions of lower concentration, and this happens without the cell expending energy. This movement arises from the constant random motion of molecules, which naturally spreads them out until equilibrium is reached. Because it relies on this natural kinetic energy, diffusion does not require cellular energy input, unlike active transport where substances are moved against their gradient using energy. So describing diffusion as spreading from high to low concentration without energy input captures the essence of how substances equilibrate in a system.