A cyclist rides at a steady pace for 30 minutes, then suddenly increases to maximum effort for a final 2-minute sprint to the finish line. During the steady pace section, the cyclist's muscles respire aerobically. During the sprint, the muscles switch to anaerobic respiration. After crossing the finish line, the cyclist continues to breathe heavily for several minutes. Explain why the muscles switch to anaerobic respiration during the sprint, and why the cyclist continues to breathe heavily after finishing.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Aerobic respiration requires a continuous supply of oxygen. During intense exercise, the rate of oxygen delivery to muscles may not keep pace with demand.
Model answer (5 marks)
During the sprint the cyclist’s muscles need ATP very rapidly, faster than aerobic respiration can provide.
Oxygen delivery to the muscles cannot keep up with this demand, so aerobic respiration cannot be sustained.
The muscles therefore switch to anaerobic respiration, which can produce ATP quickly but generates lactic acid as a by‑product.
After the sprint, the accumulated lactic acid must be removed and oxidised, which requires extra oxygen.
The cyclist’s heavy breathing supplies this extra oxygen, repaying the oxygen debt and clearing the lactic acid.
Oxygen delivery to the muscles cannot keep up with this demand, so aerobic respiration cannot be sustained.
The muscles therefore switch to anaerobic respiration, which can produce ATP quickly but generates lactic acid as a by‑product.
After the sprint, the accumulated lactic acid must be removed and oxidised, which requires extra oxygen.
The cyclist’s heavy breathing supplies this extra oxygen, repaying the oxygen debt and clearing the lactic acid.
Examiner tips
- Use the command word ‘explain’ – give a clear cause‑effect chain. Mention the rate of ATP demand, oxygen delivery, lactic acid production and oxygen debt. Show the link between heavy breathing and repayment of the oxygen debt.
- common_mistakes
- :
- Confusing anaerobic respiration with aerobic respiration. Forgetting that lactic acid is produced during anaerobic respiration. Not linking heavy breathing to the removal of lactic acid or repayment of the oxygen debt.
Mark scheme (5 marks)
- During the sprint, muscles require energy (ATP) at a faster rate than can be supplied by aerobic respiration alone
- Oxygen cannot be delivered to the muscles fast enough to maintain aerobic respiration
- Anaerobic respiration in the muscles produces lactic acid (as well as releasing energy/ATP)
- After exercise, extra oxygen is needed to break down / oxidise the lactic acid that has built up — this is the oxygen debt
- Continued heavy breathing supplies the extra oxygen needed to repay the oxygen debt / remove lactic acid
Key terms in this question
aerobic respiration · anaerobic respiration
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Cellular respiration questions
- A long-distance runner completes a marathon. Towards the end of the race, the ru…
- A student goes for a brisk walk up a steep hill. During the walk, the student's …
- A student sprints 400 metres and then rests. During the sprint, the student's mu…
- A yeast culture is kept in a sealed container with a limited supply of glucose a…