The rate of facilitated diffusion of a molecule across a membrane will increase as the concentration gradient of the molecule across the membrane increases to a certain point. Eventually, an increase in the concentration of the molecule will not cause any further increase in facilitated diffusion. This is because:

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Facilitated diffusion relies on carrier proteins in the membrane to transport molecules across the membrane. As the concentration gradient of the molecule increases, the demand for carrier proteins also increases. However, there are a limited number of carrier proteins available in the membrane. Once all the carrier proteins are in use, increasing the concentration gradient further will not result in any additional facilitated diffusion. The transport rate will reach a maximum, known as the saturation point.

The Rate of Facilitated Diffusion of a Molecule Across a Membrane