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Facilitated diffusion (facilitated transport) is a process of passive transport (diffusion) via which molecules diffuse across membranes, with the help of transport proteins (mediated transport).

Small uncharged molecules can easily diffuse across cell membranes. However, due to the hydrophobic nature of the lipids that make up cell membranes, water-soluble molecules and ions cannot do so; instead, they are helped across by transport proteins. The transport protein involved is intrinsic, that is, it completely spans the membrane. It also has a binding site for the specific molecule (e.g., glucose) or ion to be transported. After binding to the molecule, the protein changes shape and carries the molecule across the membrane, where it is released. The protein then returns to its original shape, to wait for more molecules to transport.

In contrast to active transport, facilitated diffusion does not require energy and carries molecules or ions down a concentration gradient.

Facilitated diffusion can take place in pores and gated channels. Pores never close, but gated channels open and close in response to stimuli.

The transport proteins participating in facilitated diffusion resemble enzymes. Like how enzymes are substrate specific and only catalyze certain substrates, transport proteins are solute specific and only transport certain solutes. Transport proteins also have a limit of how many solutes they can transport that they cannot exceed. Finally, molecules can inhibit the protein in a way similar to competitive inhibition in enzymes.