Facilitated diffusion uses carrier proteins to pump substances across the cell membrane that would not normally be able to cross the membrane on their own. These substances include sodium and chloride ions and molecules such as glucose.
Carrier proteins are embedded in the cell membrane. When a molecule binds to a carrier protein, the protein changes shape, allowing the substance to pass through the cell membrane. Once the substance has crossed the membrane, the carrier protein returns to its original form. Carrier proteins have roles in both facilitated diffusion and active transport, two methods of transporting items across the cell membrane.