Explain how the sodium–potassium pump (Na⁺/K⁺-ATPase) maintains the electrochemical gradient across the plasma membrane of an animal cell.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The Na⁺/K⁺‑ATPase is a transmembrane protein that hydrolyses ATP to power active transport.
1. Three Na⁺ ions are pumped out of the cell per ATP hydrolysed, lowering intracellular Na⁺ and raising extracellular Na⁺.
2. Two K⁺ ions are pumped into the cell per cycle, raising intracellular K⁺ relative to the outside.
3. Because three positive charges leave and only two enter, the pump creates a net negative charge inside the cell.
4. This net negative charge contributes to the electrical component of the electrochemical gradient (membrane potential).
1. Three Na⁺ ions are pumped out of the cell per ATP hydrolysed, lowering intracellular Na⁺ and raising extracellular Na⁺.
2. Two K⁺ ions are pumped into the cell per cycle, raising intracellular K⁺ relative to the outside.
3. Because three positive charges leave and only two enter, the pump creates a net negative charge inside the cell.
4. This net negative charge contributes to the electrical component of the electrochemical gradient (membrane potential).
Examiner tips
- Use the word 'active transport' and mention ATP hydrolysis; list the ion movements and the charge imbalance; keep the answer concise and structured.
- Show the 3:2 ratio of Na⁺/K⁺ to explain the net charge; include the effect on membrane potential.
Mark scheme (4 marks)
- The pump is a transmembrane (integral) protein that uses ATP hydrolysis to drive active transport against concentration gradients.
- Three sodium ions are pumped out of the cell per cycle, raising extracellular Na⁺ concentration and lowering intracellular Na⁺ concentration.
- Two potassium ions are pumped into the cell per cycle, raising intracellular K⁺ concentration relative to the extracellular fluid.
- Because three positive charges leave but only two enter per cycle, the pump generates a net negative charge inside the cell, contributing to the electrical component (membrane potential) of the electrochemical gradient.
Key terms in this question
Na⁺/K⁺-ATPase · electrochemical gradient
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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