Biology · Unit 2: Cell Structure and Function · 14 min read · Updated 2026-05-10
Facilitated Diffusion — AP Biology
AP Biology · Unit 2: Cell Structure and Function · 14 min read
1. What Is Facilitated Diffusion?★☆☆☆☆⏱ 3 min
Facilitated diffusion (also called facilitated transport) is a passive mechanism of membrane transport that moves polar, charged, or large hydrophilic solutes across the phospholipid bilayer, which is otherwise impermeable to these molecules. Unlike simple diffusion, it requires the assistance of specific transmembrane integral proteins to facilitate movement.
2. Channel Protein-Mediated Facilitated Diffusion★★☆☆☆⏱ 4 min
Channel proteins are transmembrane proteins that form hydrophilic pores across the phospholipid bilayer, allowing specific solutes to diffuse down their concentration gradient. Most channel proteins are highly selective: for example, voltage-gated sodium channels only allow Na⁺ ions through, blocking K⁺ ions due to size and charge matching in the pore's selectivity filter.
Aquaporins, the most abundant type of channel protein in many cells, are specific for water molecules, allowing much faster water diffusion than is possible via simple diffusion across the hydrophobic bilayer. Channel proteins can be gated (opening or closing in response to a stimulus, like a change in membrane voltage or binding of a ligand) or non-gated (always open, like most aquaporins).
Exam tip: When identifying the type of facilitated diffusion on the exam, always check for two key clues first: whether movement is regulated by an external stimulus (gating = channel) and whether energy is used (no energy = passive facilitated transport, not active).
3. Carrier-Mediated Facilitated Diffusion★★★☆☆⏱ 6 min
Carrier proteins are transmembrane proteins that bind to a specific solute on one side of the membrane, then undergo a reversible conformational change to move the solute to the other side, where it is released. Like all facilitated diffusion, movement is always down the solute’s concentration gradient, so no energy input is required.
Because each carrier protein can only bind and transport a limited number of solute molecules per unit time, carrier-mediated facilitated diffusion exhibits saturation kinetics: once all carrier binding sites are occupied, the transport rate reaches a maximum ($V_{max}$), even if the extracellular solute concentration increases further. This is a key distinguishing feature from simple diffusion, where transport rate increases linearly with solute concentration gradient indefinitely. The relationship between solute concentration and transport rate is described by the Michaelis-Menten equation:
J = \frac{V_{max}[S]}{K_M + [S]}
where $K_M$ is the solute concentration at which the transport rate is half $V_{max}$, a measure of the carrier’s affinity for the solute. A lower $K_M$ indicates a higher affinity for the solute. The most common example of carrier-mediated facilitated diffusion is the GLUT4 glucose transporter in mammalian muscle and adipose cells.
Exam tip: Saturation kinetics are a common AP exam FRQ topic: always remember that only carrier-mediated transport (and enzyme reactions) show saturation; simple diffusion and channel-mediated transport do not level off at physiological solute concentrations.
4. Comparison to Other Transport Mechanisms★★☆☆☆⏱ 3 min
The AP Biology exam regularly asks students to distinguish facilitated diffusion from simple diffusion and active transport. Compared to simple diffusion, both are passive processes that do not use ATP and move solute down the concentration gradient. The key difference is that facilitated diffusion requires a transmembrane protein, while simple diffusion occurs directly through the phospholipid bilayer.
Compared to active transport, both use membrane proteins for transport. However, active transport moves solute against the concentration gradient, requires ATP input, and (for carrier-mediated active transport) exhibits saturation kinetics. Facilitated diffusion always moves solute down the gradient and requires no energy input.
Exam tip: On any FRQ comparison question, always reference both whether movement is up/down the gradient and whether energy is required to earn full justification points.
Common Pitfalls
Why: Students remember that some water diffuses slowly through the bilayer, so they incorrectly assume all water transport is simple diffusion
Why: Students mix up the requirement for proteins with energy requirements, confusing facilitated diffusion with active transport
Why: Students generalize saturation from carrier-mediated to all types of facilitated diffusion
Why: Students associate glucose uptake with energy use in cells, so they incorrectly assume the transport itself requires ATP
Why: Students confuse regulation of opening/closing with energy input for transport against the gradient