Active transport.

The sodium-potassium pump maintains high concentrations of Na+ outside the cell and K+ inside the cell, against their concentration gradients.

  1. 3 cytoplasmic Na+ bind to the pump.

  2. ATP donates a phosphate group for energy.

  3. The protein changes its shape, expelling Na+ to the outside.

  4. 2 extracellular K+ bind to the pump, releasing the phosphate.

  5. The pump resumes its original shape.

  6. K+ is released inside.