Epithelial Solute Transport

Of all the ways in which animal cells are woven together into multicellular tissues, the epithelial arrangement is perhaps the most fascinating and fundamentally important. Epithelial sheets of cells line all external and internal surfaces in the body, and are specialized for the selective secretion and absorption of ions and organic molecules. This leads to the creation of sheltered compartments with controlled internal environments where specialized functions are performed by differentiated cells.

The laws of thermodynamic govern the direction and rate of movement of solutes across epithelial cells, i.e. down the electrochemical gradient for any molecule. The fundamental function of transporting epithelia is to generate the electrochemical gradients that will force movement of molecules in the desired direction. This is achieved by the asymmetrical distributions of transport systems (channels, ATPases, cotransporters and exchangers) in the apical and basolateral membranes of polarized epithelial cells. Primary active transport by ATPases generates electrochemical gradients that are exploited by membranes with selective permeability to produce unidirectional movement of solutes that would otherwise be thermodynamically unfavorable.