A brewery uses yeast to produce beer. The yeast is added to a sealed container containing glucose solution. After several hours, the liquid contains ethanol and bubbles of gas are observed. Explain what type of respiration the yeast is carrying out and why, describing the process that is occurring.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Yeast is a microorganism used in brewing. When sealed containers of glucose solution are inoculated with yeast, the yeast produces ethanol and a gas over several hours.
Model answer (5 marks)
The yeast is carrying out anaerobic respiration.
Because the container is sealed, no oxygen is available for aerobic respiration.
Glucose is broken down by glycolysis, producing pyruvate.
Pyruvate is then converted to ethanol and carbon dioxide.
Energy (ATP) is released from the oxidation of glucose during this process.
Because the container is sealed, no oxygen is available for aerobic respiration.
Glucose is broken down by glycolysis, producing pyruvate.
Pyruvate is then converted to ethanol and carbon dioxide.
Energy (ATP) is released from the oxidation of glucose during this process.
Examiner tips
- Use the word ‘anaerobic respiration’ early to secure the first point. Explain the lack of oxygen to justify the type of respiration. Mention glycolysis and the conversion of pyruvate to ethanol + CO₂. Show that ATP is produced to cover the energy point. Keep sentences short and use exam‑specific terms.
Common mistakes
- Confusing aerobic with anaerobic respiration. Forgetting to mention the sealed container/oxygen absence. Omitting the step of pyruvate conversion to ethanol and CO₂. Not stating that ATP is produced.
Mark scheme (5 marks)
- The yeast is carrying out anaerobic respiration
- Because the container is sealed / there is no (sufficient) oxygen available
- Glucose is broken down / used as a reactant in the process
- Ethanol (and carbon dioxide) are produced as products
- Energy is released from glucose during this process
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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