Unlike active transport, diffusion does not involve chemical energy. When molecules move (diffuse) via special transport proteinsfound within the cell membrane, it is called facilitated diffusion, otherwise it is only simple diffusion. An example of diffusion in biological system is diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane in mammalian lungs.

Diffusion may be a simple diffusion or facilitated diffusion. A simple diffusion is one in which that occurs unassisted. Facilitated diffusion in contrast is an assisted diffusion in a way that it requires a carrier molecule. For instance, polar molecules diffuse across a cell membrane through carrier molecules embedded in the cell membrane.