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Charged ions dissolve in water and diffuse through water channel proteins. These ion channels are gated so they can open and close, thus regulating the ion flow. Larger molecules diffuse through carrier proteins that change shape as the molecules are carried through, for example glucose and amino acid.
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 such as 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. Just as enzymes are substrate specific and only catalyse 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. Finally, molecules can inhibit the protein in a way similar to competitive inhibition in enzymes.
As an example of facilitated diffusion, glucose molecules diffuse by simple diffusion only very slowly across a cell membrane since glucose is not readily soluble in the phospholipid bilayer. However, glucose diffuses very quickly across a cell membrane by facilitated diffusion because the carrier proteins help the glucose molecule cross into the cell. Specific examples: GLUT1 in erythrocytes, a passive transporter involved in importing glucose molecules and GLUT2 in liver cells, involved in emporting glucose molecules.
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