Facilitated diffusion differs from simple diffusion in that it crosses a membrane with the aid of passive transport proteins embedded in the membrane. Both types of diffusion are the movement of particles in solution from areas of higher concentration to areas of lower concentration. This type of passive transport is used by cells to acquire or lose solutes that cannot penetrate their cell membranes.
All cells require materials from their environments and gain excesses of materials inside them. Many of these substances, including glucose, sodium ions and chloride ions, cannot pass through the phospholipid bilayer that constitutes cell membranes. Other substances, most notably water, pass through the membrane fairly freely. The passage of water through the membrane is another form of diffusion, known as osmosis.
Facilitated diffusion is a passive process, requiring no energy from the cell, unlike the otherwise similar process active transport. In both cases, the cell requires a protein structure, embedded in the cell membrane, to move solutes in and out of the cell. However, with active transport, the cell is moving solutes from areas of lower concentration to areas of higher concentration. This requires energy, whereas facilitated diffusion occurs without any energy input from the cell.