Mediated Transport and Ion Pumps

The penetration of water-soluble material into the cell is thought to occur via a hypothetical membrane-transport carrier, since a significant part of the cell membrane is composed of lipids and is therefore somewhat restrictive to the passive diffusion of metabolically important substances in aqueous solution. The lattice-pore model has been proposed by Naftalin (1972) as an alternative to the carrier model for sugar transport in erythrocyte membranes. However, the concept of carrier-mediated transport has been postulated by many investigators, with the general scheme consisting of three steps (1) binding of the penetrant to the carrier site (2) translocation of the penetrant across the membrane and (3) release of the penetrant on the other side of the membrane. 

The nature of the carrier represents an open question, and only its isolation and identification can provide an appropriate answer. Considering substrate specificity, which is characteristic for carrier-mediated transport, it is very likely that the substrate binding site is proteinaceous, because protein possesses selective behavior. This presumption 

Certain physiologically important iwater-soluble molecules penetrate cell membranes at a faster rate than would be expected from the knowledge of membrane structure. To explain the rapid penetration of such substances, a type of carrier-mediated transport— facilitated dif- 

The system has no requirement for free energy other than that needed to maintain the cell membrane structure. Facilitated diffusion is driven by an existing electrochemical gradient of the permeant and eventually leads to the disappearance of this gradient. 

The rate of penetration of permeant is highly stereospecific. Isomers and optical enantiomorphs of the penetrant will most likely diffuse at markedly different rates. 

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