Facilitated Diffusion is one of the form of diffusion and it is important in several metabolic processes of living cell's. Facilitated Diffusion ( also known as the facilitated transport or the passive mediated transport ) is the process of spontaneous passive transport ( as apposed to the active transport ) of molecules or ions across a biological membrane traveling through a specific transmembrane integral proteins. All the polar molecules should be transported across membranes by proteins that form a transmembrane channels. In this picture you can see that facilitated diffusion in the cell membrane, showing ion channels and carrier proteins. Molecules move from the high concentration level to the low concentration level with no energy required but are helped by carrier proteins. Polar molecules and charged ions are dissolved in water but they can not diffuse freely across cell membranes due to the hydrophobic nature of the lipids that make up the lipids bilayer.
Facilitated diffusion is the movement of large molecules across the cell membrane using proteins. Being passive, facilitated diffusion does not directly require chemical energy from the ATP hydrolysis in the transport step itself; rather, molecules and ions move down their concentration gradient. Facilitated Diffusion is different from free diffusion in several ways. First, the transport relies on molecular bending between the cargo and the membrane-embedded channel or carrier protein. Second, the rate of the facilitated diffusion is saturable with respect to the concentration between the 2 phases; unlike free diffusion which is linear in the concentration difference. Third, the temperature dependence of the facilitated transport is substantially different due to the presence of an activated binding event, as compared to free diffusion where the dependence on the temperature is mid. Polar molecules and large ions dissolved in water can not diffuse freely across the plasma membrane due to the hydrophobic. nature of the fatty acid tails of the phospholipids that make up the lipid bilayer. Only small, non-polar molecules, such as oxygen and carbon dioxide, can diffuse freely across the membrane. Therefore, non-polar molecules are transported by proteins in the form of transmembrane channels. these channels are gated, meaning that they open and close, and therefore deregulate the flow of the ions or small polar molecules across the membrane, or even sometimes against the osmotic gradient. Large molecules are transported by transmembrane carrier proteins, such as permeases, that change their conformation as the molecules are carried across(glucose or amino acids). Non-polar molecules, such as retinol or lipids, are poorly soluble in water. they are transported through the aqueous compartments of the cell or through the extracellular space in the water soluble carriers(retinol binding protein). The metabolites are not altered because no energy is required for facilitated diffusion. Only permease changes it's shape in order to transport metabolites. The form of transport through a cell membrane in which the metabolites is modified is known as the group translocation transport. Glucose, sodium ions, and chloride are just a few examples of molecules and ions that must efficiently the plasma membrane but to which the lipid bilayer of the membrane is virtually impermeable. There transport must therefore be "facilitated" by proteins that span the membrane and provide an alternative route or bypass mechanism. Various attempts have been made by engineers to copy the process of facilitated diffusion for the use of industrial scale gas and liquid separations.