A yeast cell is placed in a sealed container with a small amount of glucose solution. Explain what happens to the rate of anaerobic respiration in the yeast cell over time as the glucose is used up, and describe what waste product accumulates inside the container.

OCR GCSE Biology A: Gateway Science (J247) — B1.3 Respiration · Explain · 4 marks · View as Markdown

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

Yeast cells can respire both aerobically and anaerobically depending on the availability of oxygen. In a sealed container, the yeast quickly uses up any available oxygen.

Model answer (4 marks)

Yeast performs anaerobic respiration once oxygen is depleted.
1. Glucose is the substrate for this pathway.
2. As the glucose is consumed it becomes limiting.
3. The rate of anaerobic respiration therefore falls.
4. The waste products, ethanol and CO₂, build up in the sealed container.

Examiner tips

  • Use the word ‘anaerobic respiration’ early. Show the cause–effect chain: oxygen depletion → anaerobic respiration → glucose used → rate falls. Mention both ethanol and CO₂ as wastes.
  • Use a clear, numbered structure to match the four marking points.

Common mistakes

  • Confusing aerobic with anaerobic respiration. Failing to state that glucose is the reactant. Not naming both ethanol and CO₂ as waste products.

Mark scheme (4 marks)

  1. Yeast carries out anaerobic respiration (when oxygen runs out / in the absence of oxygen)
  2. Glucose is broken down / used as a reactant in anaerobic respiration
  3. As glucose is used up / becomes a limiting factor, the rate of anaerobic respiration decreases
  4. Ethanol (and carbon dioxide) accumulates as the waste product of anaerobic respiration in yeast

Key terms in this question

anaerobic respiration · glucose

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