A baker adds yeast to a bread dough mixture and leaves it in a warm place. After one hour, the dough has risen and smells slightly of alcohol. Explain why the dough rises and why alcohol is produced during this process.

WJEC GCSE Biology (Wales) — 1.2.1 Aerobic and 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 carries out anaerobic respiration when oxygen is scarce.
Glucose is used as the reactant in this process.
Carbon dioxide is produced and released, forming bubbles that make the dough rise.
Ethanol (alcohol) is also produced as a by‑product of the anaerobic reaction.

Examiner tips

  • Use the term ‘anaerobic respiration’ and mention oxygen absence. State glucose as the reactant. Explain CO₂ causes the rise. Mention ethanol as a second product.
  • common_mistakes
  • :
  • Confusing aerobic with anaerobic respiration. Forgetting to mention CO₂ as the cause of rising. Failing to state that ethanol is produced.

Mark scheme (4 marks)

  1. Yeast carries out anaerobic respiration (in the absence of oxygen / when oxygen runs out)
  2. Glucose is broken down / used as a reactant in anaerobic respiration
  3. Carbon dioxide is produced / released as a product of anaerobic respiration
  4. The carbon dioxide gas causes the dough to rise / expands gas bubbles in the dough; alcohol (ethanol) is also produced as a product of anaerobic respiration in yeast

Key terms in this question

yeast

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