A group of students investigated anaerobic respiration in yeast. They placed yeast and glucose solution in a sealed flask at 30 °C and collected the gas produced over 20 minutes. Explain what happens inside the yeast cells during anaerobic respiration and why the gas collected causes limewater to turn cloudy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Yeast can respire both aerobically and anaerobically. When oxygen is unavailable, yeast carries out anaerobic respiration, producing different products compared to aerobic respiration.
Model answer (5 marks)
During anaerobic respiration, yeast cells metabolise glucose in the absence of oxygen. Glucose is first converted to pyruvate via glycolysis, then pyruvate is decarboxylated to acetaldehyde and reduced to ethanol, producing NAD⁺ for the next glycolysis cycle. The main products are ethanol (alcohol) and carbon dioxide.
The gas collected is therefore mainly CO₂. When this CO₂ is bubbled through limewater (a dilute Ca(OH)₂ solution), it reacts:
CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O
The insoluble calcium carbonate precipitate makes the limewater cloudy.
The gas collected is therefore mainly CO₂. When this CO₂ is bubbled through limewater (a dilute Ca(OH)₂ solution), it reacts:
CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O
The insoluble calcium carbonate precipitate makes the limewater cloudy.
Examiner tips
- State that anaerobic respiration uses glucose and produces ethanol and CO₂; identify CO₂ as the gas collected; explain the limewater reaction to CaCO₃; keep answer concise and use correct terminology.
Common mistakes
- Confusing aerobic with anaerobic respiration; failing to mention glucose as the substrate; not recognising CO₂ as the gas that turns limewater cloudy.
Mark scheme (5 marks)
- Anaerobic respiration occurs without oxygen
- Glucose is broken down / glucose is the substrate for anaerobic respiration
- Ethanol (alcohol) and carbon dioxide are produced as products
- The gas collected is carbon dioxide
- Carbon dioxide turns limewater cloudy / reacts with limewater to form a white precipitate (calcium carbonate)
Key terms in this question
anaerobic respiration · glucose · limewater
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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