Explain how the carbon cycle helps to maintain a balance of carbon dioxide in the atmosphere, and suggest how human activity has disrupted this balance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists monitor the concentration of carbon dioxide in the atmosphere. Over the last two centuries, concentrations have risen significantly, and this rise closely follows the increase in industrialisation worldwide.
Model answer (4 marks)
The carbon cycle keeps atmospheric CO₂ in balance by
1. Plants, algae and phytoplankton absorb CO₂ during photosynthesis, converting it into organic matter.
2. When organisms respire or decompose, the stored carbon is released back as CO₂, returning it to the atmosphere.
3. Thus, natural processes normally remove and re‑introduce CO₂, keeping its concentration relatively stable.
4. Human burning of fossil fuels adds extra CO₂ that the natural cycle cannot fully absorb, so atmospheric CO₂ rises, driving climate change.
1. Plants, algae and phytoplankton absorb CO₂ during photosynthesis, converting it into organic matter.
2. When organisms respire or decompose, the stored carbon is released back as CO₂, returning it to the atmosphere.
3. Thus, natural processes normally remove and re‑introduce CO₂, keeping its concentration relatively stable.
4. Human burning of fossil fuels adds extra CO₂ that the natural cycle cannot fully absorb, so atmospheric CO₂ rises, driving climate change.
Examiner tips
- Use the word ‘photosynthesis’ and ‘respiration’ to show understanding of the cycle. Show the cause–effect link: burning → extra CO₂ → imbalance. Keep the answer concise – 4 marks, so one sentence per point.
- common_mistakes
- :
- Confusing the direction of CO₂ flow (e.g., saying plants release CO₂). Omitting the role of respiration/decomposition as the return step. Not linking the extra CO₂ to the rise in concentration and climate change.
Mark scheme (4 marks)
- The carbon cycle removes carbon dioxide from the atmosphere through photosynthesis (by plants/algae/phytoplankton)
- Carbon dioxide is returned to the atmosphere through respiration and/or decomposition of organisms
- Burning/combustion of fossil fuels releases additional carbon dioxide into the atmosphere
- This additional carbon dioxide cannot be fully removed by the natural cycle, so the concentration of carbon dioxide in the atmosphere increases, contributing to climate change
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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