Facilitated diffusion is a process by which molecules and ions are transported from one location to another with the aid of some intermediary, such as a protein. In biology, diffusion usually refers to the movement of molecules or ions across a membrane, though it can also occur through liquid-filled compartments in biological systems. Transport proteins are the most common intermediaries that help ions and molecules to reach their destinations. Facilitated diffusion is also referred to as passive-mediated transport or facilitated transport.
The name "passive-mediated transport" gives an important insight into the nature of this process: it is a passive form of transportation, meaning that it uses no energy. In active transportation, chemical energy is expended to move a substance against a concentration gradient. Despite the necessity for an intermediary, no energy expenditure is necessary in this type of diffusion. The protein intermediaries are generally needed because the polarity or charge of certain molecules and ions prevents them from freely crossing membranes. Cell membranes are made up of phospholipid bilayers that prevent substances with particular polarities from passing though.
Only small molecules with minimal charge and polarity are able to diffuse freely across a cell's plasma membrane. Polar molecules are almost always incapable of doing so without the aid of proteins. These proteins can form transmembrane channels, which are essentially tunnels through the membrane, and these "tunnels" are gated so they can selectively allow or prevent various ions and molecules from passing through the membrane.
Larger molecules cannot necessarily fit through the transmembrane channels formed by some proteins, so others, called membrane transport proteins, are available to help them to cross. Protein carriers open at one end to accept a molecule or ion then open on the other end to release it. Sometimes, enzymes within transmembrane channels or membrane transport proteins speed the progression of the molecule or ion that is passing through, helping it to overcome the resistances that impede it from crossing the membrane successfully.
Facilitated diffusion may also occur across aqueous, or water-filled, spaces in cells. Some non-polar molecules, particularly some large organic molecules, are insoluble and have difficulty moving through water. Water soluble proteins are able to bind to the molecules and carry them across the cell. Some of these proteins actually change their shape in order to optimize themselves for the specific molecule or ion that they are transporting.