facilitated diffusion

This term describes a specific mechanism of passive transport. It is distinguished from simple diffusion by the requirement of a helper molecule, such as a channel or carrier protein, to move polar or charged substances across the hydrophobic lipid bilayer of a cell membrane.

Because it is a passive process, it relies entirely on the concentration gradient and does not require ATP. It is frequently contrasted with active transport, which moves substances against their gradient and requires energy. In academic and medical contexts, it is used to explain how essential nutrients like glucose enter cells efficiently.