01Proteins as doors
Facilitated diffusion is passive movement down a gradient through a protein. Two types: channel proteins (tunnels, often gated, like aquaporins and ion channels) and carrier proteins (change shape to shuttle a molecule, like GLUT glucose transporters).
Because the number of proteins is limited, facilitated diffusion saturates: once every transporter is busy, adding more solute doesn't speed things up.
02Why graphs matter
Plot rate vs. concentration: simple diffusion is a straight line that keeps rising; facilitated diffusion rises then plateaus at the transporter maximum.
Gated ion channels open in response to signals (voltage, a ligand). Nerve impulses are Na⁺ rushing through voltage-gated channels — still passive, because it's down the gradient.
03Notebook box
Plateau on a transport graph = carrier proteins saturated.
Worked example: Glucose uptake levels off at high glucose. Adding ATP does not raise it. Explain. → Uptake is facilitated diffusion limited by the number of GLUT carriers, not energy.