A renewable energy company is developing a biofuel made from fermented plant material. The biofuel contains ethanol. Explain why ethanol produced from plants may be considered carbon neutral, and describe one limitation of replacing petrol with this biofuel on a large scale.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Biofuels such as ethanol can be produced by fermenting sugars from crops such as sugar cane. Some scientists argue these fuels are carbon neutral, while others point out practical drawbacks of large-scale use.
Model answer (5 marks)
Plants absorb CO₂ during photosynthesis, storing it as carbon in their biomass.
When the plant material is fermented to produce ethanol and the ethanol is later burnt as fuel, the CO₂ released is essentially the same CO₂ that the plants had absorbed.
Thus the amount of CO₂ emitted by burning the ethanol is balanced by the amount that was taken up during plant growth – giving a net zero change in atmospheric CO₂, so the fuel can be regarded as carbon neutral.
However, producing large amounts of ethanol requires vast areas of farmland to grow the feedstock crops.
This land demand competes with food production and can lead to food shortages, habitat loss or deforestation, limiting the feasibility of replacing petrol on a large scale.
When the plant material is fermented to produce ethanol and the ethanol is later burnt as fuel, the CO₂ released is essentially the same CO₂ that the plants had absorbed.
Thus the amount of CO₂ emitted by burning the ethanol is balanced by the amount that was taken up during plant growth – giving a net zero change in atmospheric CO₂, so the fuel can be regarded as carbon neutral.
However, producing large amounts of ethanol requires vast areas of farmland to grow the feedstock crops.
This land demand competes with food production and can lead to food shortages, habitat loss or deforestation, limiting the feasibility of replacing petrol on a large scale.
Examiner tips
- Use the term ‘carbon neutral’ and explain the CO₂ cycle; mention photosynthesis and combustion; give a specific limitation and explain its impact; keep answer concise and use correct terminology
Common mistakes
- Confusing the amount of CO₂ released with the amount absorbed; not explaining the land‑use limitation; writing a vague or unrelated limitation
Mark scheme (5 marks)
- Plants absorb/take in carbon dioxide from the atmosphere during photosynthesis (as they grow)
- When ethanol/biofuel is burned it releases carbon dioxide back into the atmosphere
- The CO₂ released on burning equals/is balanced by the CO₂ absorbed during growth, so there is no net increase in atmospheric CO₂
- A valid limitation stated — e.g. large areas of land/farmland are needed to grow the crops (so less land for food production)
- Limitation developed/explained — e.g. this could lead to food shortages or destruction of habitats/deforestation
Key terms in this question
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