Practice question

Na+/K+ ATPase is a heterotetrameric P-type primary active pump that couples ATP hydrolysis to export of three Na+ and import of two K+ per catalytic cycle, maintaining steep ionic gradients of low intracellular Na+ around 12 mM and high K+ around 140 mM. These gradients generate a negative membrane potential and provide chemical potential stored as sodium motive force. Numerous secondary active transporters harness that force rather than ATP directly, including sodium-coupled amino acid transporters SNAT and B0AT systems in intestine and proximal tubule, as well as SGLT1 glucose cotransporters. Sodium entry down its electrochemical gradient drives uphill accumulation of solutes essential for protein synthesis and intermediary metabolism. Without continuous pumping, intracellular Na+ accumulates, cell swelling occurs, calcium handling via Na+/Ca2+ exchange reverses, action potential repolarization fails, and concentrative nutrient uptake collapses, demonstrating why primary pumping is indispensable for absorption, volume control, and neuronal excitability beyond mere ion homeostasis. This paradigm is central for competitive examinations testing secondary active transport and nutrient absorption physiology.