This review presents the theory of solute transport across frog skin, epithelial cells of the intestine, the kidney tubules, and other similar systems. This theory is a part of a broader theory of the living cell, called the association-induction hypothesis.
The central pumping mechanism is the cyclic activity of a sponge-like cytoplasmic protein(s), which alternately sop-up (by adsorption) the solute being transported and squeeze it out again (by desorption) into the cytoplasmic water at a high concentration level. The uptake phase begins with the adsorption of ATP on key cardinal sites of the protein involved; the release phase is triggered by the desorption of ATP through its dephosphorylation during a transitory activation of an ATPase.
The theory recognizes the different nature of the two surfaces of the epithelial cells and assigns to each a key role in the active transport. The surface facing the "source solution" has a higher permeability to the solute being transported, while the surface facing the "sink solution" has a low permeability to the solute. This asymmetry in permeability insures that the solute sopped up by the cytoplasmic protein(s) comes primarily from the source solution. Depolarization of the water of the cell surface facing the sink solution (but not that facing the source solution) insures that the solute released into the cytoplasmic water during the squeezing phase leaves the cell only through the opposite surface as that where the solute has entered the cell.
Ling, Gilbert N.
"Theory of Active Transport Across Frog Skin and Other Bifacial Cell Systems: A Subsidiary of the Association-Induction Hypothesis,"
Scanning Microscopy: Vol. 4
, Article 20.
Available at: https://digitalcommons.usu.edu/microscopy/vol4/iss3/20