facilitated diffusion
/ fuh-SIL-ih-tay-tid dih-FYOO-zhun /
Some things that a cell desperately needs — like glucose, the sugar that fuels you — cannot squeeze through the oily membrane on their own, because they are too large and too water-loving. They are like a crowd waiting outside a locked glass door: there is room on the other side, and they want to go, but the door simply will not let them through. The cell solves this by installing helper proteins that act as doors built just for them.
Facilitated diffusion is diffusion that needs a protein helper. A specific molecule binds to a channel or carrier protein, which provides a water-lined passage or briefly cradles the molecule and flips it across. Crucially, the protein only helps the flow go the way it naturally wants to — downhill, from high concentration to low — and the cell spends no energy on it. The protein does not push; it just opens a path that the oily core was blocking.
Because it relies on a limited number of helper proteins, facilitated diffusion has a speed limit that simple diffusion does not: pile on more and more of the substance and the rate climbs, then levels off once every protein is busy, like a checkout line that cannot go faster than its open registers. This is still passive transport — no energy used — but it is selective and saturable, which lets the cell control exactly which large or charged molecules get a free ride across.
Glucose entering most of your body's cells rides in through carrier proteins called GLUT transporters — pure downhill diffusion, but impossible without those helper doors.
A helper protein opens the door; gravity-like diffusion does the rest.
Despite needing a protein, facilitated diffusion uses no energy and only moves things downhill — it is not the same as active transport, which can push uphill against the gradient.