Ethanol can be produced by fermentation. Describe the process of fermentation and explain why ethanol produced this way is considered a more sustainable fuel than petrol.

AQA A-Level Chemistry (7405) — 3.3.11 Amines (A-Level only) · Describe and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Fermentation is a biological process that uses glucose (sugar) as the starting material. Yeast cells act as a catalyst, with enzymes in the yeast breaking down glucose into ethanol and carbon dioxide. The reaction occurs anaerobically, i.e. without oxygen. Petrol is produced from crude oil, which is a finite, non‑renewable resource. The glucose used in fermentation is derived from plants, which are renewable and can be regrown, making ethanol a more sustainable fuel than petrol.

Examiner tips

  • Use the exact terms ‘yeast’, ‘enzymes’, ‘anaerobically’ and ‘finite/non‑renewable’
  • Show the sequence: glucose → yeast → enzymes → ethanol + CO₂
  • Explain the sustainability link: renewable plant source vs non‑renewable crude oil

Common mistakes

  • Confusing fermentation with combustion or mislabeling yeast as a product rather than a catalyst
  • Omitting the anaerobic condition or the renewable nature of the plant source

Mark scheme (5 marks)

  1. Fermentation uses sugar/glucose (solution) as the starting material
  2. Yeast is used / yeast acts as a catalyst / enzymes in yeast break down glucose
  3. The process occurs without oxygen / anaerobically
  4. Petrol is made from crude oil, which is a finite/non-renewable resource
  5. Sugar/glucose used in fermentation comes from plants, which are renewable / can be regrown, making ethanol a more sustainable fuel

Key terms in this question

fermentation · ethanol

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