Facilitated diffusion is passive transport in which a membrane protein allows a solute to move down its existing thermodynamic driving force without directly consuming ATP or another energy source.

The membrane protein changes the rate at which the solute crosses the membrane; it does not reverse the direction favored by the gradient.

For an uncharged solute, the driving force is mainly its concentration difference. If glucose is more concentrated outside a cell than inside and a suitable glucose transporter is present, net transport can proceed inward until the concentration-driven tendency is reduced.

For an ion, both concentration difference and the electrical potential difference across the membrane contribute to the direction of passive movement.

Channel-mediated facilitated diffusion

An open channel provides a hydrophilic pathway through the membrane. Selected ions or water can pass rapidly through while moving in the direction favored by the conditions on the two sides.

Opening or closing a gated channel regulates permeability. It does not, by itself, turn the channel into an active pump.

Carrier-mediated facilitated diffusion

A carrier can bind a solute on one side, change conformation and release it on the other.

Because a finite number of carriers cycle at finite rates, carrier-mediated transport can saturate. Increasing solute concentration eventually produces progressively smaller increases in transport rate once most carriers are occupied.

Example

Suppose a membrane is nearly impermeable to glucose but contains a glucose carrier. If extracellular glucose concentration exceeds intracellular concentration, the carrier can greatly accelerate net glucose entry. If the concentrations become equal and no other driving force exists, the carrier continues microscopic exchange in both directions but produces no sustained net flux.

Facilitated diffusion is therefore distinct from both simple diffusion through the lipid bilayer and active transport. It uses a protein pathway but still follows a pre-existing thermodynamic driving force.