How does the intensity of facilitated diffusion vary in relation to the concentration of the moved substance? What is the limiting factor?

Like simple diffusion facilitated diffusion is more intense when the concentration gradient of the substance enhances and less intense when the gradient lessens. In facilitated diffusion though there is a limiting factor: the quantity of the permeases that facilitate the transport by the membrane. Even in a situation in which the concentration gradient of the diffusing substance enhances, if there are not enough permeases to perform the transport there will be no enhance in the intensity of the diffusion. This situation is known as saturation of the transport proteins and it represents the point at which the maximum transport capacity of the substance across the membrane is achieved.