A student investigates the rate of respiration in yeast. She mixes yeast with glucose solution and collects the gas produced. Explain why gas is produced and describe what happens to the glucose inside the yeast cells during anaerobic respiration.

AQA A-Level Biology (7402) — 3.5.2 Respiration · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Anaerobic respiration occurs when oxygen is absent.

In yeast, glucose is broken down by glycolysis to pyruvate.

Because oxygen is lacking, pyruvate is converted to ethanol (alcohol) and carbon dioxide.

The carbon dioxide released is the gas that bubbles out and is collected.

Examiner tips

  • Use the term "anaerobic respiration" first to show understanding of the condition.
  • Explain the conversion of glucose to ethanol and CO₂ in one clear sentence.
  • Mention that CO₂ is the gas collected.
  • Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

  • Confusing aerobic with anaerobic respiration.
  • Failing to state that CO₂ is the gas collected.
  • Using vague terms like "breakdown" without specifying ethanol and CO₂.

Mark scheme (5 marks)

  1. Anaerobic respiration occurs in the absence of oxygen
  2. Glucose is broken down / converted during anaerobic respiration
  3. Glucose is converted into ethanol (alcohol) in yeast
  4. Carbon dioxide is also produced during anaerobic respiration in yeast
  5. The carbon dioxide produced is the gas that is collected / causes bubbles

Key terms in this question

anaerobic respiration · glucose · yeast

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