Na/K-ATPase exchanges ions how?

• A. 2 Na+ out, 3 K+ in
• B. 3 Na+ out, 2 K+ in
• C. 3 Na+ in, 2 K+ out
• D. 2 Na+ in, 3 K+ out

Answer: (B)

The main idea tested is the exact ion movement and direction of the Na+/K+-ATPase. This pump uses one ATP to move 3 sodium ions out of the cell and 2 potassium ions into the cell, against their gradients. This stoichiometry makes the pump electrogenic—net positive charge is exported each cycle—which helps establish and maintain the resting membrane potential and the respective Na+ and K+ gradients. Mechanistically, the pump binds 3 Na+ on the cytosolic side, gets phosphorylated by ATP, changes shape to release Na+ outside, then binds 2 K+ from outside, is dephosphorylated, returns to its original conformation, and releases K+ inside. Other patterns, such as moving more Na+ in or moving ions in the opposite direction, do not reflect how the pump operates.