In the sodium-potassium pump, which ion goes into the cell and which exits?

• A. Sodium is carried in, potassium is carried out
• B. Potassium is carried in, sodium is carried out
• C. Both in
• D. Both out

Answer: (B)

The key concept is how the Na+/K+-ATPase pump uses energy to move ions against their gradients, establishing and maintaining the cell’s ion balance. In this pump, three sodium ions are exported to the extracellular space, and two potassium ions are imported into the cytoplasm for each cycle. So the ion that goes into the cell is potassium, while sodium exits the cell. The process relies on ATP hydrolysis to drive a conformational change: Na+ binds inside, ATP is used to phosphorylate the pump, Na+ is released outside, K+ binds from outside, the pump is dephosphorylated, and K+ is released inside, returning to the original state. This active transport helps keep intracellular Na+ low and K+ high, which is essential for maintaining the resting membrane potential and cell volume.