During a long-distance cycling race, a competitor's muscles must sustain contraction for several hours. At the start of the race, the cyclist's muscles respire aerobically. As the race intensifies, the muscles begin to respire anaerobically. Explain how muscle cells obtain energy for contraction during both aerobic and anaerobic respiration, and describe what happens in the body after the race to deal with the products of anaerobic respiration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A competitive cyclist completes a gruelling four-hour race. During the final sprint finish, the cyclist pushes their leg muscles to maximum effort. After crossing the finish line, the cyclist continues to breathe rapidly and deeply for several minutes.
Model answer (5 marks)
Aerobic respiration: glucose is oxidised in mitochondria using O₂, producing CO₂, H₂O and ~36 ATP, which powers actin–myosin cross‑bridge cycling.
Anaerobic respiration: in the absence of O₂, glucose is partially oxidised in the cytoplasm to lactate (lactic acid) via lactate dehydrogenase, yielding only 2 ATP per glucose.
After the race the accumulated lactate is transported to the liver and kidneys where it is converted back to glucose (Cori cycle) or oxidised in mitochondria, a process that requires O₂. This extra oxygen demand is the oxygen debt.
The cyclist’s rapid, deep breathing increases O₂ uptake to repay the debt and restore ATP levels.
Anaerobic respiration: in the absence of O₂, glucose is partially oxidised in the cytoplasm to lactate (lactic acid) via lactate dehydrogenase, yielding only 2 ATP per glucose.
After the race the accumulated lactate is transported to the liver and kidneys where it is converted back to glucose (Cori cycle) or oxidised in mitochondria, a process that requires O₂. This extra oxygen demand is the oxygen debt.
The cyclist’s rapid, deep breathing increases O₂ uptake to repay the debt and restore ATP levels.
Examiner tips
- Use the terms ‘aerobic’ and ‘anaerobic’ and mention mitochondria and lactate dehydrogenase. Show the energy yield difference (≈36 ATP vs 2 ATP). Explain the oxygen debt and link it to the post‑exercise breathing pattern.
- common_mistakes
- :
- Confusing lactic acid with lactic acid’s ionised form; students often write ‘lactic acid’ instead of ‘lactate’. Forgetting to mention the Cori cycle or that lactate is oxidised in mitochondria. Not linking the rapid breathing to the oxygen debt explicitly.
Mark scheme (5 marks)
- Aerobic respiration breaks down glucose using oxygen to release energy / ATP for muscle contraction
- During anaerobic respiration, glucose is broken down without oxygen
- Anaerobic respiration in animal / muscle cells produces lactic acid
- After exercise, lactic acid must be broken down / oxidised, which requires extra oxygen — this is the oxygen debt
- The cyclist continues to breathe rapidly / deeply after the race to take in the extra oxygen needed to repay the oxygen debt
Key terms in this question
aerobic respiration · anaerobic respiration
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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