Flexi Says: Diffusion is the net movement of a substance from an area of high concentration to an area of low concentration. Diffusion happens in nonliving systems as well as living cells and it does not require any external energy input. It happens in liquids and gases because their particles move randomly from place to place. Diffusion is an important process for living things; it is how simple substances such as oxygen and carbon dioxide move in and out of cells.
