Facilitated Diffusion (College Board AP® Biology): Study Guide
Facilitated diffusion
Facilitated diffusion can be defined as:
the movement of large, polar molecules and ions across membranes via transport proteins; this occurs down a concentration gradient and requires no energy input
Specialized transport or channel proteins are required for facilitated diffusion
These proteins are highly specific, allowing the transport of only one type of substance
The transport proteins that allow facilitated diffusion can be either:
channel proteins: pores that allow the passage of substances
carrier proteins: proteins with a binding site on one side of a membrane that change shape to release the transported substance on the other side
Examples of facilitated diffusion include:
passage of water via transport proteins known as aquaporins
transport of ions, including Na+ and K+
Examiner Tips and Tricks
Be careful not to confuse facilitated diffusion with active transport. Both processes can involve carrier proteins, but facilitated diffusion involves movement down a concentration gradient, and so does not require an energy input, while active transport involves movement against a gradient, and so requires energy from ATP.
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