Food production: micro-organisms and fish farming — Edexcel International GCSE Biology
How yeast and Lactobacillus are used to make bread and yoghurt, how fermenters work, and how fish are farmed.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Yeast is a single-celled fungus used to make bread because it respires anaerobically, breaking down glucose into ethanol and carbon dioxide. In warm dough, bubbles of carbon dioxide are trapped and make the dough rise. Baking kills the yeast, expands the gas bubbles so the bread is light, and evaporates the ethanol.
This practical measures the rate of anaerobic respiration in yeast from the volume of carbon dioxide made in a set time. Yeast is mixed with boiled, cooled sugar solution under a layer of oil to keep oxygen out. The rate rises with temperature up to an optimum of about 35–40 °C, then falls as enzymes denature.
Yoghurt is made by adding the bacterium Lactobacillus to pasteurised, cooled milk and incubating it at about 40–45 °C. The bacteria respire the milk sugar lactose and make lactic acid. The acid lowers the pH, making the milk proteins clump so the milk thickens and tastes sour, and stops many other micro-organisms growing.
An industrial fermenter is a large container in which micro-organisms are grown in large numbers to make a product such as a food, medicine or enzyme. Conditions are controlled for fast growth: aseptic precautions, nutrients, optimum temperature using a cooling water jacket, optimum pH, sterile air for oxygenation, and stirring paddles for agitation.
Fish farming
Biology onlyFish farming raises large numbers of fish, such as salmon, in tanks, ponds or net cages to provide protein. Farmers maintain water quality, control intraspecific predation by keeping fish of the same size together, control interspecific predation by keeping out other species, control disease, remove waste, control feeding and use selective breeding so fish grow quickly.
Exam questions
8 exam-style questions (26 marks), each with its mark scheme.
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16 cards: flip them, mark what you knew, and practise the rest.
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The whole of use of biological resources on one page, so you can see where this subtopic fits.
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Method, variables, risks and a results table, with questions to answer.
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Quick check questions
What gas makes bread dough rise?
Show answer
Carbon dioxide, from anaerobic respiration in yeast.
Write the word equation for anaerobic respiration in yeast.
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Glucose → ethanol + carbon dioxide.
Which bacterium is used to make yoghurt, and what does it make from lactose?
Show answer
Lactobacillus; it makes lactic acid.
Why are aseptic precautions used in a fermenter?
Show answer
To stop unwanted micro-organisms getting in and competing with or spoiling the product.
Biology only Give one way of controlling disease on a fish farm.
Show answer
Any one from: clean water, vaccines, medicines, low stocking density.
Frequently asked questions
What is anaerobic respiration in yeast?
Anaerobic respiration in yeast is respiration without oxygen, also called fermentation. Yeast breaks down glucose to make ethanol and carbon dioxide, releasing energy: glucose → ethanol + carbon dioxide. Bread making uses this, because the carbon dioxide bubbles trapped in the dough make it rise, and the ethanol evaporates during baking.
How is yoghurt made using bacteria?
Yoghurt is made by adding Lactobacillus bacteria to milk that has been pasteurised and cooled, then incubating it at about 40–45 °C. The bacteria turn the milk sugar lactose into lactic acid. The acid lowers the pH, making the milk proteins clump so the milk thickens, and stops many other micro-organisms growing.
Why are conditions controlled in an industrial fermenter?
Conditions are controlled so the micro-organisms grow and make the product as quickly as possible. Aseptic precautions stop unwanted microbes competing or spoiling the product. Nutrients supply materials for growth, the optimum temperature and pH let enzymes work fastest, sterile air allows aerobic respiration, and stirring spreads nutrients, oxygen and heat evenly.
How do fish farmers increase the yield of fish?
Biology only Fish farmers increase yield by controlling conditions so fish grow quickly and survive. They maintain water quality, keep fish of the same size together to reduce intraspecific predation, keep out other species to stop interspecific predation, control disease, remove waste, control feeding and selectively breed fish that grow fastest.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026