Yeast cells are used in bread making. Explain how yeast cells produce carbon dioxide when making bread dough rise.

Eduqas GCSE Biology — 1.2.1 Aerobic and anaerobic respiration · Explain · 4 marks · View as Markdown

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

Bread dough is made by mixing flour, water, sugar and yeast. The dough is left in a warm place before baking. During this time the dough increases in volume.

Model answer (4 marks)

Yeast cells carry out anaerobic respiration because the dough contains little oxygen. They metabolise glucose, which is present in the flour and added sugar, to produce energy. In this process glucose is broken down to give carbon dioxide and ethanol as the main products. The carbon dioxide is released as a gas and becomes trapped in the gluten network of the dough, causing the dough to expand and rise.

Examiner tips

Use the term "anaerobic respiration" and mention oxygen limitation
Show that glucose is the reactant and CO₂ is a product
Explain that CO₂ is trapped in the dough to cause rise
Keep the answer concise and use exact terminology

Common mistakes

Confusing aerobic with anaerobic respiration
Omitting that glucose is the reactant
Failing to mention that CO₂ is trapped in the dough

Mark scheme (4 marks)

Yeast carries out anaerobic respiration (in the absence of oxygen / when oxygen is limited)
Glucose is broken down / used as a reactant
Carbon dioxide (and ethanol/alcohol) are produced as products
The carbon dioxide gas becomes trapped in the dough, causing it to expand / rise

Key terms in this question

carbon dioxide