Facilitated diffusion is a form of passive transport that utilizes transport proteins. These transport proteins are embedded within cell membranes, serving as figurative highways for molecules to get in and out of the cell.
Facilitated diffusion is also sometimes called carrier-mediated transport. Like conventional diffusion, facilitated diffusion requires a concentration gradient, a region of high concentration of a chemical species connected by a transport medium to a region of lower concentration. The species in both regions move randomly, but because the higher concentration region has more of the species, there is net migration from the higher concentration region to the low one.
The transport proteins in facilitated diffusion act as a specialized transport medium, allowing certain chemical species to move more efficiently from the high concentration region to the low one. This process is vital to life, as it enables cells to obtain essential nutrients including lipids, simple sugars, water and oxygen.
The rate at which species move from the high concentration region is a complex function of the difference in concentration between the two regions, the thermal and chemical energy available to the species, species-carrier interactions and absolute temperature. All passive transport processes generally proceed faster as temperature increases.