Explain how the combustion of fossil fuels and the process of photosynthesis affect the concentration of carbon dioxide in the atmosphere.

Edexcel GCSE Biology (1BI0) — 9.3 Material cycles · Explain · 4 marks · View as Markdown

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

Carbon dioxide is continuously entering and leaving the atmosphere through a range of processes. Human activities have significantly altered the balance of carbon dioxide in the atmosphere over recent centuries.

Model answer (4 marks)

Combustion of fossil fuels releases carbon dioxide into the atmosphere, increasing its concentration. This occurs because the carbon stored in fossil fuels is oxidised and released as carbon dioxide gas. Photosynthesis removes carbon dioxide from the atmosphere, decreasing its concentration. During photosynthesis, carbon dioxide is converted into organic molecules such as glucose, incorporating carbon into living organisms.

Examiner tips

Use the exact phrases "increases its concentration" and "decreases its concentration"
Show the cause–effect link: oxidation releases CO₂, photosynthesis consumes CO₂
Mention the products of photosynthesis (glucose/biomass) to demonstrate understanding

Common mistakes

Confusing combustion with photosynthesis, or vice versa
Failing to state that combustion releases CO₂ or that photosynthesis removes it
Omitting the role of carbon being incorporated into biomass

Mark scheme (4 marks)

Combustion of fossil fuels releases carbon dioxide into the atmosphere, increasing its concentration
This occurs because the carbon stored in fossil fuels is oxidised and released as carbon dioxide gas
Photosynthesis removes/absorbs carbon dioxide from the atmosphere, decreasing its concentration
During photosynthesis, carbon dioxide is converted into organic molecules/glucose/biomass, incorporating carbon into living organisms

Key terms in this question

combustion · photosynthesis · carbon dioxide