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

Naomi Holyoak

Written by: Naomi Holyoak

Reviewed by: Ruth Brindle

Updated on

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+

Diagram of a phospholipid bilayer showing an ion channel with open and closed states, allowing Na+ movement from the exterior to the cytoplasm.
Channel proteins are pores that allow the diffusion of specific substances across membranes; some may be gated, meaning that they can be open or closed depending on conditions
Diagram of a phospholipid bilayer showing carrier proteins facilitating polar molecules from high to low concentration across the membrane.
Carrier proteins change shape to allow substances to move from one side of a membrane to the other

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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Naomi Holyoak

Author: Naomi Holyoak

Expertise: Biology

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.

Ruth Brindle

Author: Ruth Brindle

Expertise: Biology

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.