A yeast culture is kept in a sealed container with a limited supply of glucose and no oxygen. Explain what happens to the rate of cellular respiration in the yeast cells over time as the glucose is used up, and describe how the products of respiration differ from those produced when oxygen is available.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.1 Cellular respiration · Explain · 5 marks · View as Markdown

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

Yeast can respire both aerobically and anaerobically depending on the availability of oxygen and glucose.

Model answer (5 marks)

1. In the absence of oxygen the yeast carries out anaerobic respiration.
2. Anaerobic respiration converts glucose into ethanol and carbon dioxide.
3. When oxygen is present the same glucose is oxidised in aerobic respiration to give carbon dioxide and water.
4. As the limited glucose is consumed the amount of substrate available for respiration falls.
5. Consequently the rate of cellular respiration decreases over time as glucose is depleted.

Examiner tips

  • Use the term "anaerobic respiration" first, then state the products (ethanol + CO₂).
  • Contrast the products with aerobic respiration (CO₂ + H₂O).
  • Explain the decline in rate by linking it to glucose depletion.

Mark scheme (5 marks)

  1. Without oxygen, yeast respires anaerobically
  2. Anaerobic respiration in yeast produces ethanol (alcohol) and carbon dioxide
  3. When oxygen is available, aerobic respiration produces carbon dioxide and water (not ethanol)
  4. As glucose is used up, the rate of respiration decreases / slows down
  5. Glucose is the reactant / substrate for respiration, so as it is depleted there is less substrate available for respiration to occur

Key terms in this question

glucose · cellular respiration

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