Protein pump moving Na+ out, K+ into cells.

• A. Sodium-Chloride Cotransporter
• B. Sodium-Calcium Exchanger
• C. Sodium-Potassium Pump
• D. Proton Pump

Answer: (C)

The key idea is an energy-dependent membrane pump that creates and maintains the cell’s sodium and potassium gradients. The protein that moves Na+ out of the cell while bringing K+ in uses energy from ATP hydrolysis to drive this transport. It exchanges 3 sodium ions leaving the cell for 2 potassium ions entering, across the plasma membrane. Because more positive charge is pumped out than in, it helps make the inside of the cell more negative, contributing to the resting membrane potential and supporting various secondary active transport processes.

This pump is distinct from others: a sodium-chloride cotransporter moves Na+ and Cl− together without exporting Na+; a sodium-calcium exchanger typically moves Na+ and Ca2+ in opposite directions to regulate calcium; and a proton pump moves H+ ions to acidify compartments or generate proton gradients, not to establish the Na+/K+ gradient.