Facilitated Diffusion (College Board AP® Biology) : Study Guide

Facilitated diffusion

• The structure of a molecule affects its ability to pass through the plasma membrane

• Specialized membrane proteins are required to allow the passive transport of large, polar molecules and ions across the bilayer; this is facilitated diffusion

  • These proteins are highly specific, allowing the transport of only one type of substance

• The transport proteins that allow facilitated diffusion can either be:

  • 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⁺