Ileal absorptive cell. Multiple brush border transporters couple ion influxes (Na⁺ and, in one instance, H⁺) to organic solute influxes or exchange one ion for another. Basolateral-membrane carriers facilitate diffusion of organic solutes and are not coupled to ion movements. Na/K-ATPase in the basolateral membrane uses energy from ATP hydrolysis to drive Na⁺ extrusion and K⁺ uptake (3:2 stoichiometry), both moving against their electrochemical gradients. K⁺ and Cl^– channels in the basolateral membrane open in response to cell swelling and elevations of intracellular Ca²⁺. Some of the cellular H⁺ and HCO₃^– extruded in exchange for Na⁺ and Cl^– is provided through the action of carbonic anhydrase (CA). Additional HCO₃^– may enter the cell through the basolateral membrane NHE1 or the Na(HCO₃)₃ cotransporter (not shown). The cell electric potential is 30–40 mV negative relative to the lumen, providing an electric, as well as a chemical, driving force for Na⁺ entry via Na–organic solute cotransport. G, glucose or galactose; AA, amino acid (there are actually several amino acid carriers); BA, bile acid anion; OP, oligopeptide; OPase, oligopeptidase (almost all oligopeptides entering the cell intact are quickly hydrolyzed).