Passive Transport (continued)

It consists of passage of solute molecules from the highest to the lowest concentrationuntil you strike a balance.

It is a slow process except when the concentration gradient is too high or the distances traveled are short. The passage of substances across the membrane occurs in response to the concentration gradient.

Certain substances enter the cell in favor of the concentration gradient and without energy expenditure, but at a speed greater than that allowed by simple diffusion.

This occurs, for example, with glucose, some amino acids and certain vitamins. The rate of facilitated diffusion is not proportional to the concentration of the substance. Increasing the concentration reaches a saturation point from which the input follows simple diffusion. This suggests the existence of a carrier molecule called permease in the membrane.

When all permeases are being used, the speed cannot increase. Since some different solutes may compete for the same permease, the presence of one makes it difficult for the other to pass.