A baker uses yeast to make bread dough rise. The yeast is mixed into the dough and left in a warm place. After some time, bubbles of gas are produced and the dough expands. Explain why the dough rises, including the name of the process responsible and the conditions inside the dough that cause this process to occur.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Yeast are single-celled fungi that can respire both aerobically and anaerobically. When mixed into bread dough, the yeast cells are surrounded by flour, water and sugar.
Model answer (5 marks)
The yeast in the dough carries out anaerobic respiration.
1. Inside the dough there is very little oxygen, so aerobic respiration cannot occur.
2. The yeast breaks down sugar (glucose) into pyruvate.
3. Pyruvate is then converted to ethanol and CO₂.
4. CO₂ is released as a gas.
5. The CO₂ bubbles get trapped in the dough, causing it to expand and rise.
1. Inside the dough there is very little oxygen, so aerobic respiration cannot occur.
2. The yeast breaks down sugar (glucose) into pyruvate.
3. Pyruvate is then converted to ethanol and CO₂.
4. CO₂ is released as a gas.
5. The CO₂ bubbles get trapped in the dough, causing it to expand and rise.
Examiner tips
- Use the term "anaerobic respiration" and mention CO₂ as the key gas.
- Show the sequence: low O₂ → glucose → CO₂ + ethanol → bubbles.
- Keep the answer short and to the point; each point should match a mark scheme element.
Common mistakes
- Confusing aerobic with anaerobic respiration; forgetting to mention low oxygen.
- Failing to state that CO₂ is the gas that causes the rise.
- Using vague language like "the yeast makes the dough rise" without explaining the process.
Mark scheme (5 marks)
- The yeast respires anaerobically (inside the dough)
- There is little/no oxygen available inside the dough (so aerobic respiration cannot occur)
- Sugar/glucose is broken down (by the yeast)
- Carbon dioxide is produced (as a product of anaerobic respiration in yeast)
- The carbon dioxide gas causes the dough to expand/rise (by forming bubbles)
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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