Facilitated Diffusion

Facilitated Diffusion — Diffusion With Help

Facilitated diffusion is still diffusion in the sense that it only ever moves a substance downhill, from high concentration to low — it cannot create a gradient, only speed up movement along one that already exists. What makes it different from simple diffusion is that certain substances cannot cross the lipid membrane unassisted and need a helper.

Why some molecules need help: the plasma membrane is built mainly of lipid, so small, lipid-soluble molecules slip through easily and quickly. Larger molecules, and especially molecules with a water-loving (hydrophilic) character, cannot dissolve into the lipid interior on their own. For these, specific membrane transport proteins provide a binding site and a route across.

Properties of this assisted route:

• Highly selective — the cell controls exactly which molecules it lets in.
• Saturable — once every transport protein is occupied, the rate cannot increase further, no matter how steep the gradient.
• Sensitive to inhibitors — molecules that block the protein's binding site can stop transport. …