Diffusion

Diffusion is the spreading of particles, such as gas molecules or dissolved substances, from where they are more concentrated to where they are less concentrated. It needs no energy from the cell: the particles' own random thermal motion carries them down the concentration gradient. Once the concentration is even, molecules keep moving, but there is no longer any net flow in either direction, a state called dynamic equilibrium.

Diffusion is a form of passive transport. Small nonpolar molecules such as oxygen and carbon dioxide diffuse straight through the lipid bilayer of the cell membrane, while ions and polar molecules must pass through channel or carrier proteins, a process called facilitated diffusion. The rate increases with a steeper gradient, a higher temperature, a larger surface area and smaller, lighter molecules, and it drops quickly as the distance grows.

Because diffusion is slow over long distances, it limits how large a cell can be. In the body, it drives gas exchange in the lung alveoli, where oxygen moves into the blood and carbon dioxide moves out.