Facilitated Diffusion

Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is a process of passive transport (as opposed to active transport), with this passive transport aided by integral membrane proteins. Facilitated diffusion is the spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins. The facilitated diffusion may occur either across biological membranes or through aqueous compartments of an organism. Polar molecules and charged ions are dissolved in water but they cannot diffuse freely across the plasma membrane due to the hydrophobic nature of the fatty acid tails of phospholipids that make up the lipid bilayers. Only small nonpolar molecules, such as oxygen can diffuse easily across the membrane. All polar molecules are transported across membranes by proteins that form transmembrane channels. These channels are gated so they can open and close, thus regulating the flow of ions or small polar molecules. Larger molecules are transported by transmembrane carrier proteins, such as permeases that change their conformation as the molecules are carried through, for example glucose or amino acids. Polar molecules, such as retinol or lipids are poorly soluble in water. They are transported through aqueous compartments of cells or through extracellular space by water-soluble carriers as retinol binding protein. The metabolites are not changed because no energy is required for facilitated diffusion. Only permease changes its shape in order to transport the metabolites. The form of transport through cell membrane which modifies its metabolites is the group translocation transportation.

Glucose, sodium ions and chloride ions are just a few examples of molecules and ions that must efficiently get across the plasma membrane but to which the lipid bilayer of the membrane is virtually impermeable. Their transport must therefore be "facilitated" by proteins that span the membrane and provide an alternative route or bypass.

Various attempts have been made by engineers to mimic the process of facilitated transport in synthetic (i.e., non-biological) membranes for use in industrial-scale gas and liquid separations, but these have met with limited success to date, most often for reasons related to poor carrier stability and/or loss of carrier from the membrane.

Other articles related to "facilitated diffusion":

Glucose Uptake - Facilitated Diffusion
... GLUT1 and GLUT3 are located in the plasma membrane of cells throughout the body, as they are responsible for maintaining a basal rate of glucose uptake ... Basal blood glucose level is approximately 5mM ...
Carrier Protein - Physiology - Facilitated Diffusion
... Facilitated diffusion is the passage of molecules or ions across a biological membrane through specific carrier proteins and requires no energy ... Facilitated diffusion is used especially in the case of large polar molecules and charged ions once such ions are dissolved in water they cannot diffuse freely across cell membranes due to the ... The type of carrier proteins used in facilitated diffusion is slightly different than in active transport ...