Explain why a muscle cell that is actively contracting requires a continuous supply of reduced NAD (NADH), and describe what happens to the reduced NAD produced during glycolysis when oxygen is unavailable.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During intense exercise, skeletal muscle cells can continue contracting even when oxygen delivery is insufficient to meet the demand of aerobic respiration. Under these conditions, the cell must rely on anaerobic pathways to sustain ATP production.
Model answer (4 marks)
Reduced NAD⁺ is the electron carrier that must be regenerated for glycolysis to keep producing ATP. In aerobic cells it is oxidised in the electron transport chain, which also produces ATP.
When oxygen is lacking the electron transport chain stops, so NADH cannot be oxidised and NAD⁺ becomes unavailable.
The cell therefore uses lactate dehydrogenase: the NADH from glycolysis donates its hydrogen to pyruvate, forming lactate.
This reaction regenerates NAD⁺, allowing glycolysis – and thus ATP production – to continue during intense exercise.
When oxygen is lacking the electron transport chain stops, so NADH cannot be oxidised and NAD⁺ becomes unavailable.
The cell therefore uses lactate dehydrogenase: the NADH from glycolysis donates its hydrogen to pyruvate, forming lactate.
This reaction regenerates NAD⁺, allowing glycolysis – and thus ATP production – to continue during intense exercise.
Examiner tips
- State the role of NAD⁺ in glycolysis and the ETC. Explain why the ETC cannot operate without O₂. Describe the lactate‑dehydrogenase reaction. Show how NAD⁺ is regenerated to sustain ATP production.
Common mistakes
- Confusing NAD⁺ with NADH as the required species. Saying the ETC works without oxygen. Forgetting that lactate dehydrogenase regenerates NAD⁺. Not linking the regeneration of NAD⁺ to continued ATP synthesis.
Mark scheme (4 marks)
- Reduced NAD is required as a hydrogen/electron carrier to be oxidised in the electron transport chain, regenerating NAD for use in glycolysis (and the Krebs cycle)
- Without oxygen, the electron transport chain cannot function, so reduced NAD cannot be oxidised by this route and NAD becomes unavailable
- Reduced NAD from glycolysis donates its hydrogen to pyruvate, reducing pyruvate to lactate (catalysed by lactate dehydrogenase)
- This process regenerates oxidised NAD, allowing glycolysis to continue and ATP to continue being produced
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Respiration questions
- Explain why the rate of ATP production in a cell is significantly reduced when a…
- A student claims that the electron transport chain is responsible for most of th…
- Explain why a muscle cell that is actively contracting continues to produce lact…
- Explain why the rate of ATP production in a cell would decrease significantly if…