2.7 Facilitated Diffusion

Passive, but with help

Facilitated diffusion is passive transport (no ATP, moves down the gradient) that uses transport proteins to ferry molecules that can't cross the bilayer alone — like glucose, ions, and other polar or charged molecules.

Channels and carriers

Channel proteins form a tunnel through the membrane; aquaporins are water channels. Carrier proteins bind a molecule and change shape to shuttle it across. Both are specific to the molecules they move, but neither uses energy.

Key terms

Facilitated diffusion

Passive transport of polar/charged molecules through proteins, down the gradient.

Channel protein

A protein forming a pore that lets specific molecules or ions pass through.

Carrier protein

A protein that binds a molecule and changes shape to move it across.

Aquaporin

A channel protein that lets water cross the membrane quickly.

Passive but protein-mediated

Facilitated diffusion needs a protein but no energy.

Try an AP-style question

What best explains the plateau?

1. The membrane becomes impermeable to glucose
2. All the carrier proteins are working at maximum capacity (saturated)
3. The glucose gradient reverses above 10 mM
4. Glucose begins crossing by simple diffusion instead