Mechanisms of transcellular transport.

Transcellular transport of physiologically important substances across cellular membranes (epithelia) is achieved through two distinct membrane processes in series, the apical and basolateral membrane steps, and the paracellular pathways. In cases of active transport, only one of the membrane processes is uphill. Uphill processes can be divided into three categories according to the mode of. energy utilization; primary, secondary and tertiary. The direction of transcellular transport (absorption or secretion) is determined by the location and direction of the uphill membrane transport.Transcellular transport of nutrients by epithelia is coupled with Na+. Na-coupled cotransport mechanisms (secondary active processes) in the apical membranes take up the respective substrates in a uphill manner causing intracellular accumulation of the substrates, which exit across the basolateral membranes along their gradients formed. Molecular properties and carrier mechanisms of the cotransport has partly been elucidated. For Na+or Ca++, a primary active process at the basolateral membrane is responsible for transcellular active transport, whereas active transport of Cl-, phosphate, SO4- is secondary active process coupled with Na+. In some cell types, 2Cl-/Na+/K+ cotransport is responsible for active Cl- transport. Characteristics of these different types of ion transport is to be outlined.