Practice question

Primary role of Na+/K+ ATPase in context of secondary active transport is generation and maintenance of electrochemical sodium gradient that powers numerous symporters and antiporters. By pumping three Na+ outward and two K+ inward per ATP hydrolyzed, it creates chemical gradient low intracellular Na+ and electrical gradient interior negative near minus 70 mV. Combined sodium motive force represents stored free energy. Solute carrier families SLC5 SGLT, SLC6 neurotransmitter transporters, SLC38 amino acid transporters, SLC9 NHE and SLC8 NCX exploit this force allowing Na+ to move downhill while accumulating glucose, amino acids, phosphate or extruding H+ and Ca2+ uphill. Without pump activity Na+ accumulates intracellularly, gradient collapses, concentrative nutrient absorption in gut and kidney fails, pH regulation falters, calcium overload occurs via reverse Na+/Ca2+ exchanger. Ouabain inhibition illustrates dependence. Thus pump does not directly carry glucose but indirectly energizes secondary systems, explaining coupling between primary and secondary active transport in animal cells for nutrient uptake and homeostasis.