Examples of facilitated diffusion are the passing of K+ ions through a membrane with an aid of a potassium transport protein and the passing of glucose and amino acids with the aid of proteins called permeases. Retinol binding protein acts as a water-soluble carrier for retinol and fatty acids.
Diffusion is movement of molecules across a membrane. There are three main types of diffusion: simple, channel and facilitated types. Particles normally move from areas of high concentration to areas of low concentration along the concentration gradient. Prokaryotic cells demonstrate simple diffusion, whereas facilitated diffusion only happens in more complex eukaryotic cells.
Simple diffusion requires no energy and happens linearly. The more particles added to the solution, the higher the concentration becomes. Channel diffusion involves proteins letting molecules through a membrane.
In a facilitated diffusion, an integral protein changes its conformation to let a passing molecule through, as in the case of permeases proteins. Other protein carriers, such as retinol binding proteins, do not change. Carrier or transport proteins stay in their places in the membrane. Transport proteins are tailored to let only particular molecules through.
During facilitated diffusion, unlike other types, individual molecules can travel against the concentration gradient, but the net movement is as in other types of diffusion. As opposed to simple diffusion, saturation can be reached during facilitated diffusion.