Explain why a muscle cell that is actively contracting continues to produce lactate even when oxygen is available.

OCR A-Level Biology A (H420) — 5.7 Respiration · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

During intense exercise, skeletal muscle cells can produce lactate despite being supplied with oxygen via the bloodstream. This observation puzzled early physiologists who expected aerobic respiration to dominate whenever oxygen was present.

Model answer (4 marks)

During intense contraction the ATP demand rises above the maximum rate at which mitochondria can generate ATP by aerobic respiration. Consequently, anaerobic glycolysis runs in parallel to supply ATP rapidly. In this process glucose is converted to pyruvate faster than the mitochondria can oxidise it, so the excess pyruvate is reduced to lactate. This conversion regenerates NAD⁺, which is required for glycolysis to continue producing ATP.

Examiner tips

Use the word ‘excess’ to show why pyruvate is diverted to lactate. Mention that lactate production regenerates NAD⁺, a key point. Show the link between high ATP demand and the limited rate of aerobic ATP production.

Common mistakes

Confusing lactate with lactic acid. Saying that lactate is produced only when oxygen is absent. Forgetting to mention the regeneration of NAD⁺ as the reason for lactate formation.

Mark scheme (4 marks)

The rate of ATP demand during intense contraction exceeds the maximum rate at which aerobic respiration can supply ATP
Anaerobic glycolysis (lactate fermentation) runs simultaneously to produce additional ATP quickly
Pyruvate is produced faster than the mitochondria can accept and oxidise it, so excess pyruvate is reduced to lactate
Lactate production regenerates NAD+, allowing glycolysis to continue producing ATP

Key terms in this question

lactate