The sodium and potassium pump uses energy from ATP to move three sodium ions (Na⁺) out of a cell and two potassium ions (K⁺) into it during each cycle. Both ions are transported against their concentration gradients, so this is active transport.
The pump is an integral membrane protein and an exchange transporter. Its binding sites change their affinity for Na⁺ and K⁺ as phosphorylation causes the protein to change conformation.
| Stage | Mechanism |
|---|---|
| 1. Sodium binds | Three Na⁺ from the cytoplasm bind to specific sites on the pump. |
| 2. ATP is hydrolysed | ATP is hydrolysed to ADP and phosphate. The phosphate attaches to the pump, phosphorylating it. |
| 3. Sodium is released | Phosphorylation causes a conformational change, exposing the binding sites to the extracellular side. The three Na⁺ are released outside the cell. |
| 4. Potassium binds | Two extracellular K⁺ bind to the pump. |
| 5. Phosphate is released | K⁺ binding causes dephosphorylation and another conformational change. |
| 6. Potassium is released | The pump returns to its original shape and releases two K⁺ into the cytoplasm. |
Because three positive ions leave for every two that enter, there is a net movement of one positive charge out of the cell. The pump is therefore electrogenic. It helps establish Na⁺ and K⁺ concentration gradients and contributes to the generation and maintenance of membrane potentials, particularly in neurons.
A common misconception is that Na⁺ and K⁺ move through the pump by facilitated diffusion. They do not: facilitated diffusion is passive and occurs down a concentration gradient, whereas the pump uses ATP to transport both ions against their gradients.
Exam technique: This is AHL content in B2.1.15. For an “explain” question, state the 3 Na⁺:2 K⁺ ratio, ATP hydrolysis, phosphorylation, conformational changes, movement against concentration gradients, and the link to membrane potential.
