A baker adds yeast to dough in a warm, enclosed container. Over time, the dough rises and a distinctive smell develops. Explain how the process of anaerobic respiration in yeast is responsible for these observations.

Cambridge International IGCSE Biology (0610) — 12.3 Anaerobic respiration · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Yeast cells in the dough carry out anaerobic respiration, breaking down glucose to produce carbon dioxide and ethanol.
The CO₂ released forms bubbles in the dough, causing it to rise.
The ethanol produced gives the dough a distinctive alcoholic smell.

Examiner tips

  • Use the word ‘breaks down’ to show the reaction; mention CO₂ and ethanol explicitly.
  • Explain the role of CO₂ in rising and ethanol in the smell – both are required for full marks.

Common mistakes

  • Confusing aerobic respiration with anaerobic; forgetting to mention CO₂ or ethanol.
  • Describing the smell as ‘sweet’ or ‘yeasty’ without linking it to ethanol.

Mark scheme (4 marks)

  1. Yeast respires anaerobically, breaking down glucose to produce carbon dioxide (and alcohol).
  2. The carbon dioxide gas produced causes the dough to rise.
  3. Alcohol (ethanol) is also produced during anaerobic respiration in yeast.
  4. The distinctive smell is due to the alcohol (ethanol) produced.

Key terms in this question

anaerobic respiration · yeast

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