Facilitated Diffusion

Facilitated diffusion is a passive process. Specific protein carriers or channels are used. Charged particles and polar molecules are transported across the membrane by facilitated diffusion, as are large molecules that are too big to cross the membrane by simple diffusion. Movement is still from a region of high concentration to a region of lower concentration.

Co-transport is where two different particles are transported through the same protein at the same time. A good example of co-transport is seen with glucose and Na⁺, which are transported into cells together.

In addition to increasing temperature, the rate of facilitated diffusion is dependent on the number of protein carriers. The number of protein carriers limits the rate of facilitated diffusion, as a maximum rate of diffusion is reached when there is a high concentration of the substance that is being transported. This means all the protein carriers are in use (or full/saturated), as shown on the graph.