Use yeast anaerobic respiration in bread and biofuel production
Yeast anaerobic respiration produces ethanol and carbon dioxide; carbon dioxide raises bread dough, while ethanol can be used as a biofuel.
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Know that genes are sections of DNA and that organisms use enzymes in cellular processes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What are the products of anaerobic respiration in yeast?
What you will learn
- Describe anaerobic respiration by yeast during ethanol production for biofuels
- Explain how carbon dioxide from yeast raises bread dough
Use yeast fermentation to make ethanol
When yeast respires anaerobically, it breaks down glucose and produces ethanol and carbon dioxide. In biofuel production, the ethanol is collected and can be used as a fuel. Yeast needs a suitable supply of sugar and conditions that support fermentation.
Anaerobic means that oxygen is not used for this respiration pathway. Do not describe the process as complete oxidation: ethanol remains an energy-containing product.
Explain why dough rises
In bread-making, yeast respires anaerobically using sugars in the dough. The carbon dioxide produced forms gas bubbles that expand the dough, making it rise. During baking, much of the ethanol evaporates.
The bread-making and biofuel examples use the same yeast products but use them differently: carbon dioxide is useful for dough expansion, and ethanol is collected as the biofuel product.
Pause and explain
Why does bread dough rise when yeast respires anaerobically?
Worked example
A sealed fermentation vessel contains yeast and a sugar solution. Name the products and identify the product collected for biofuel.
Show the worked solution
- Recall the yeast anaerobic products.
- Distinguish fuel ethanol from carbon dioxide.
Answer Yeast produces ethanol and carbon dioxide. The ethanol is collected for biofuel production.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the role of yeast anaerobic respiration in producing ethanol biofuel.
Give me a hint
Name the substrate pathway and the collected product.
Compare my reasoning
- Yeast respires anaerobically using sugar.
- It produces ethanol and carbon dioxide; ethanol is collected.
Yeast anaerobically respires sugar, producing ethanol and carbon dioxide. The ethanol is collected and used as a biofuel.
Look for these in your work
- Anaerobic yeast respiration stated
- Ethanol product and use identified
Explain how yeast makes bread dough rise.
Give me a hint
Follow carbon dioxide from respiration to gas bubbles.
Compare my reasoning
- Yeast anaerobically respires sugars in the dough.
- Carbon dioxide bubbles expand the dough.
Yeast respires sugars anaerobically and produces carbon dioxide. The gas bubbles expand the dough, causing it to rise.
Look for these in your work
- Carbon dioxide source stated
- Expansion explained
A student says the gas released during bread-making is the ethanol fuel. Correct the statement.
Give me a hint
Distinguish the two products and their uses.
Compare my reasoning
- Carbon dioxide is the gas.
- Ethanol is the liquid product collected for biofuel.
Yeast produces both carbon dioxide and ethanol. Carbon dioxide is the gas that makes dough rise; ethanol is the product collected for biofuel production.
Look for these in your work
- Products distinguished
- Uses matched
Common mistakes
- Saying yeast anaerobic respiration produces only ethanol
- Claiming carbon dioxide is the biofuel
- Forgetting the gas bubbles cause dough expansion
What can you explain now?
What are the products of anaerobic respiration in yeast?
Compare with the explanation
Ethanol and carbon dioxide
Both products are made.
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Make it stick
What does yeast make during anaerobic respiration, and how are the products used in bread-making and biofuel production?
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