When certain large molecules are unable to move through the membrane without the help of channel proteins
Explanation:
Facilitated diffusion occurs when large molecules that are unable to pass through a membrane on their own make use of channel proteins to move from one side of the membrane to the other. Facilitated diffusion can occur when a liquid on one side of a membrane has a higher osmotic pressure than a liquid on the other side of the membrane, but it isn’t a required feature of facilitated diffusion. Similarly, facilitated diffusion can occur when there’s a large membrane potential present, but it isn’t a required feature of facilitated diffusion. Facilitated diffusion doesn’t require the use of energy, so the presence of ATP is irrelevant to facilitated diffusion occurring.
