Explain why the products of fermentation by yeast can eventually stop the fermentation process from continuing.

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).

Yeast is used in brewing to produce alcoholic drinks. During brewing, yeast breaks down glucose anaerobically in a process called fermentation. Over time, the fermentation process slows and eventually stops.

Model answer (4 marks)

1. Fermentation by yeast produces ethanol as a product.
2. Ethanol accumulates in the surrounding medium.
3. High ethanol concentrations denature the enzymes that catalyse fermentation.
4. Denatured enzymes can no longer speed up glucose breakdown, so fermentation stops.

Examiner tips

Use the exact terms "ethanol", "denature", "enzymes", "catalyse".
Show the causal chain: product → accumulation → enzyme denaturation → cessation.
Keep each point short and to the point to match the 4‑mark structure.

Common mistakes

Confusing fermentation with aerobic respiration.
Using vague terms like "stops" without linking to enzyme denaturation.
Omitting the role of ethanol accumulation.

Mark scheme (4 marks)

Fermentation produces alcohol (ethanol) as a product
Alcohol accumulates / builds up in the surroundings of the yeast
High concentrations of alcohol denature the enzymes responsible for (catalysing) fermentation / respiration
Denatured enzymes can no longer catalyse / speed up the breakdown of glucose, so fermentation stops

Key terms in this question

fermentation