Yeast can respire anaerobically. Explain why yeast respiration slows down and eventually stops when the temperature rises above 35 °C.

Cambridge International IGCSE Biology (0610) — 12.1 Respiration · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Yeast respiration is carried out by enzymes.

At temperatures above 35 °C the enzymes become denatured.

Denaturation changes the shape of the active site, or permanently alters it.

With the active site altered the enzymes can no longer catalyse the reactions, so the rate of respiration falls and eventually stops.

Examiner tips

  • Use the term ‘enzymes’ and ‘denatured’ – these are key words.
  • Explain that denaturation changes the active site shape – this shows understanding of the mechanism.
  • Mention that the reaction rate falls and stops – links cause and effect.
  • Keep the answer concise and to the point to fit the 4‑mark limit.

Common mistakes

  • Using ‘heat’ instead of ‘temperature’ or ‘high temperatures’ – examiners look for the specific 35 °C threshold.
  • Failing to mention the active site change – the mark scheme rewards this detail.
  • Writing a long explanation or extra information that distracts from the main points.

Mark scheme (4 marks)

  1. Respiration in yeast is catalysed by enzymes
  2. High temperatures above the optimum cause the enzymes to denature
  3. The shape of the active site changes / is permanently altered
  4. Therefore the rate of respiration decreases / stops as the reactions can no longer be catalysed

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