Explain why yeast cells switch to anaerobic respiration when oxygen is no longer available, and describe the products that are released during this process.

Pearson Edexcel International GCSE Biology (4BI1) — 2.7 Respiration · Explain · 4 marks · View as Markdown

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

Yeast is a single-celled fungus used in bread-making and brewing. When yeast is provided with a plentiful supply of glucose, it respires. However, the availability of oxygen can change depending on the conditions in which the yeast is kept.

Model answer (4 marks)

When oxygen is absent, yeast cannot carry out aerobic respiration, so it switches to anaerobic respiration to obtain energy from glucose.

Anaerobic respiration still releases ATP from glucose, allowing the cell to survive.

During this process yeast produces ethanol (alcohol) as a waste product.

It also releases carbon dioxide, which is expelled from the cell.

Examiner tips

Use the word ‘switches’ to show change; mention oxygen absence first. Show that energy is still produced. Name both ethanol and CO₂ as products. Keep answer to 4 short points to match marks.

Common mistakes

Confusing fermentation with respiration; not naming both ethanol and CO₂. Using vague terms like ‘some energy’ without specifying ATP. Adding unnecessary details such as the role of yeast in baking.

Mark scheme (4 marks)

Yeast switches to anaerobic respiration because oxygen is unavailable / absent (so aerobic respiration cannot occur)
Anaerobic respiration still releases energy (from glucose) for the yeast cell
Ethanol (alcohol) is produced during anaerobic respiration in yeast
Carbon dioxide is also produced during anaerobic respiration in yeast

Key terms in this question

anaerobic respiration