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Yeast: biofuels and bread

Biotechnology and genetic modification · Biotechnology · note 2 of 4

Spec 21.2.1, 21.2.2Download all Biotechnology notes (PDF)Download all notes (PDF)6 pages

Yeast: biofuels and breadSpec 21.2.1, 21.2.2

In short

Yeast is a single-celled fungus that respires anaerobically when there is no oxygen, breaking down glucose to make ethanol and carbon dioxide. In biofuel production, the ethanol is separated from the tank and used as a renewable fuel. In bread-making, carbon dioxide bubbles are trapped in the dough and make it rise; the ethanol evaporates during baking.

Yeast is a single-celled fungus. When there is no oxygen it respires anaerobically, breaking down glucose to make ethanol (alcohol) and carbon dioxide. This is called fermentation.

glucose → alcohol (ethanol) + carbon dioxide
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Biofuels

Sugar from plants such as sugar cane, or sugar made by breaking down starch from maize, is fed to yeast in a closed tank with no oxygen. The yeast respires anaerobically and makes ethanol. The ethanol is separated and used as a biofuel, for example mixed with petrol to fuel vehicles. The plants can be regrown, so it is a renewable fuel.

Bread-making

  1. Yeast is mixed with flour, sugar and water to make dough, and the dough is kept warm.
  2. The yeast respires anaerobically and makes carbon dioxide (and ethanol).
  3. The carbon dioxide forms bubbles that are trapped in the dough, which makes the dough rise.
  4. When the bread is baked, the gas bubbles expand, the yeast is killed and the dough sets with air spaces. The ethanol evaporates.
Common mistake:

It is the carbon dioxide that makes bread rise, not the ethanol. Write that the yeast respires anaerobically, not just that it 'ferments'.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

Why are bacteria used in biotechnology?

Bacteria are used in biotechnology because they have a rapid reproduction rate, so a large population and a large amount of product are made in a short time. They can also make complex molecules, such as proteins, that are difficult to make in other ways. Once genetically modified, they can make human proteins such as insulin.

How does yeast make bread rise?

Yeast makes bread rise by respiring anaerobically in the warm dough and producing carbon dioxide. The carbon dioxide forms bubbles that are trapped in the dough, which makes it rise. When the bread is baked, the bubbles expand, the yeast is killed and the dough sets with air spaces. The ethanol evaporates.

How do biological washing powders work?

Biological washing powders contain enzymes that digest stains into small, soluble molecules that wash out. Proteases break down protein stains such as blood or egg into amino acids, lipases break down fats into fatty acids and glycerol, and amylases break down starch into sugars. They work well at low temperatures such as 30 to 40 °C.

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