IGCSE Biology / Using microorganisms
Yeast, anaerobic respiration and bread
Edexcel International GCSE Biology (4BI1), chapter 21: Using microorganisms, specification point 5.5, 5.6. Free lesson.
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Yeast is a single-celled fungus that can respire with or without oxygen. Its anaerobic respiration produces carbon dioxide, which makes bread rise, and ethanol, which evaporates during baking.
What yeast cells are like
Yeast is a single-celled fungus. Its cell structures help explain how it lives and respires. The chitin cell wall identifies yeast as a fungus, not a plant. The cell membrane surrounds the cytoplasm, where many cell reactions happen. A nucleus means yeast is a eukaryote, unlike bacteria. Yeast has mitochondria, so it can also respire aerobically when oxygen is available. A vacuole stores food and other materials.
How yeast reproduces
Yeast reproduces by budding, a form of asexual reproduction. A culture grown from one yeast cell is therefore made up of genetically identical cells called clones.
What anaerobic respiration produces
When yeast respires without oxygen, it breaks down glucose and produces ethanol and carbon dioxide. This is the same reaction used in bread making and in making alcoholic drinks. Glucose → ethanol + carbon dioxide No oxygen is used in this reaction.
How yeast makes bread rise
Yeast, flour, water and sugar are mixed and kneaded into dough. In a warm place, the yeast respires anaerobically and uses the sugar. Carbon dioxide gets trapped as bubbles in the stretchy gluten network. As the bubbles expand, the dough rises. The dough is baked, and the heat kills the yeast so respiration stops. Baking sets the bubbles in place in the cooked dough. The ethanol evaporates in the oven, so the bread is not alcoholic.
How to measure fermentation rate
You can compare fermentation rates by measuring the volume of gas made in a fixed time. Keep other conditions the same so temperature is the factor being tested. The rate rises as temperature increases, up to an optimum of about 30 to 40 °C. Above the optimum, the rate falls because the yeast enzymes denature.