Which substance does NOT require energy for its diffusion?

Diffusion is a passive process that involves the movement of molecules from an area of higher concentration to an area of lower concentration, driven solely by the concentration gradient. This means that substances naturally spread out in an attempt to reach equilibrium without any input of energy. Molecules such as oxygen and carbon dioxide typically move across cell membranes in this manner.

This distinguishes diffusion from active transport, which requires cellular energy (usually in the form of ATP) to move substances against their concentration gradient. Facilitated diffusion also allows molecules to pass through the cell membrane but requires specific transport proteins to assist the movement; however, like simple diffusion, it does not require energy. Cellular respiration, on the other hand, is a metabolic process that does consume energy.

Therefore, diffusion is correctly identified as the process that does not require energy, as it relies on the natural kinetic movement of molecules down their concentration gradient.