Facilitated diffusion is the passive movement of molecules or ions down their concentration gradient through specific membrane proteins. Like simple diffusion, it uses no ATP; the proteins simply provide a route through the hydrophobic core of the membrane.It is used by ions and polar molecules, which cannot pass through the bilayer, for example sodium ions, chloride ions, glucose and amino acids. Two types of protein are involved:Channel proteins form water-filled, hydrophilic pores. Each is specific to one ion or molecule. Many are gated: they open or close in response to a signal, such as a change in voltage across the membrane or the binding of a chemical. Aquaporins are channel proteins for water.Carrier proteins have a binding site specific to one molecule, such as glucose. The molecule binds, the carrier changes shape, and the molecule is released on the other side of the membrane. The direction of net movement depends only on the concentration gradient.Because there is a limited number of proteins in the membrane, the rate of facilitated diffusion rises with concentration difference at first, then levels off when all the proteins are working at full speed. The process is saturated. Simple diffusion shows no such plateau.