Bread dough rises when yeast is added to a flour and water mixture. Explain why the dough rises, including the role of respiration in this process.

AQA A-Level Biology (7402) — 3.5.2 Respiration · Explain · 5 marks · View as Markdown

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

Bread makers add yeast to a mixture of flour and water. After a period of time in a warm environment, the dough swells and increases in volume.

Model answer (5 marks)

Yeast cells in the dough carry out anaerobic respiration, using glucose from the flour as a reactant. During this process glucose is broken down to produce carbon dioxide and ethanol (alcohol). The carbon dioxide is released as a gas and becomes trapped in the gluten network of the dough. The trapped gas expands the dough, causing it to rise and increase in volume.

Examiner tips

  • Use the term "anaerobic respiration" and mention glucose as reactant
  • Show that CO₂ is a product and is trapped in the dough
  • Explain that trapped CO₂ causes expansion
  • Mention ethanol as a co‑product

Common mistakes

  • Failing to state that yeast performs anaerobic respiration
  • Confusing aerobic respiration with the process in dough
  • Not linking CO₂ to the rise of the dough

Mark scheme (5 marks)

  1. Yeast carries out anaerobic respiration
  2. Glucose is broken down / used as a reactant
  3. Carbon dioxide is produced as a product of anaerobic respiration in yeast
  4. Ethanol / alcohol is also produced (alongside carbon dioxide)
  5. Carbon dioxide gas gets trapped in the dough and causes it to expand / rise

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