What is the sodium-potassium pump?

• A. Moves Na+ in and K+ out using ATP
• B. Moves Ca2+ in and out using ATP
• C. Moves Na+ out and K+ in using ATP
• D. Pumps H+ across membrane

Answer: (C)

The sodium-potassium pump is an active transporter that uses energy from ATP to move ions against their gradients, helping maintain the cell’s resting membrane potential and ion balance. In each cycle, it binds three Na+ ions from the inside, uses one ATP molecule to drive a conformational change that ejects those Na+ to the outside, then binds two K+ ions from the outside and returns them to the inside. The net outcome is three Na+ ions pumped out and two K+ ions pumped in, powered by ATP, which also makes the pump electrogenic.

This is why the best description is the one that states Na+ is moved out and K+ is moved in using ATP. Other options describe different ions (Ca2+ or H+) or reversed directions that don’t match the actual Na+/K+ pump mechanism.