4.1.5 Yeast in Food Production — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions

Part 1 of 8Key ideas

Key ideas

Photosynthesis

Photosynthesis carbon dioxide + water → glucose + oxygen; 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Anaerobic respiration in yeast

Anaerobic respiration in yeast glucose → ethanol + carbon dioxide; C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

Lactic fermentation (bacteria)

Lactic fermentation (bacteria) glucose/lactose → lactic acid.

Bread

Bread yeast CO₂ makes dough rise; ethanol evaporates during baking; warmth speeds yeast enzymes.

Paraffin heaters in glasshouses burn fuel, raising both temperature and CO₂ concentration.

Glasshouse CO₂ is often raised to about 0.1% (from 0.04%).

Percentage increase in yield = (new yield − old yield) ÷ old yield × 100.

Rate of yeast respiration can be measured as volume of CO₂ ÷ time (cm³/min).

Fish farms

Fish farms control water quality, feed quality, predators, disease (antibiotics), intraspecific predation, and use selective breeding.

Part 2 of 8Learning objectives

1

Describe how glasshouses and polythene tunnels increase crop yield by controlling limiting factors.

2

Evaluate the use of fertilisers, pesticides and biological control.

3

Describe the use of yeast in bread and beer/wine making and Lactobacillus in yoghurt production.

4

Describe the use of industrial fermenters and the conditions they control.

5

Explain methods used in fish farming to produce high yields.

Part 3 of 8Essential knowledge

1

In glasshouses, temperature can be raised by heaters, CO₂ increased by paraffin burners, and light extended with artificial lighting, so photosynthesis is faster and yield greater.

2

Polythene tunnels are cheaper, trap heat, protect crops from weather and pests, and extend the growing season.

3

Increasing a factor only increases yield while it is limiting; beyond that, extra cost gives no benefit, so growers balance cost against extra yield.

4

Nitrate is needed for amino acids/proteins; lack causes stunted growth. Manure releases nutrients slowly and improves soil structure; artificial fertiliser acts quickly but can leach and cause eutrophication.

5

Pesticides kill pests and raise yields but can kill harmless organisms, accumulate in food chains (bioaccumulation) and select for resistant pests.

6

Biological control uses a natural enemy (e.g. ladybirds eat aphids); it is specific and leaves no residues, but is slow, does not eliminate pests completely and the agent may become a pest itself.

7Bread

Bread yeast, sugar, flour and water are mixed and kept warm; yeast respires anaerobically, producing CO₂ that makes the dough rise; baking kills yeast and evaporates ethanol.

8Beer/wine

Beer/wine yeast ferments sugars (from malted barley or grapes) anaerobically to produce ethanol; air is excluded so anaerobic respiration occurs.

9Yoghurt

Lactobacillus converts lactose into lactic acid, which lowers pH, thickens the milk and gives a sour taste that also helps stop spoilage.

10

Industrial fermenters are sterilised and supplied with nutrients, sterile air, a water-cooled jacket, pH and temperature probes and stirrers for optimum microbial growth.

11Fish farming

Fish farming fish kept in nets/tanks, water quality controlled (oxygen, waste removed), high-protein food given in controlled amounts, predators excluded, disease treated with antibiotics or reduced by lower density, fish separated by size to prevent intraspecific predation,

and selective breeding for fast growth.

12Environmental concerns of fish farms

Environmental concerns of fish farms waste and uneaten food can cause eutrophication; disease and parasites can spread to wild fish.

13Magnesium ions

Plants need magnesium ions to make chlorophyll; a deficiency causes yellowing of the leaves (and less photosynthesis). Nitrate deficiency gives weak, stunted growth with yellow leaves.

14Glasshouses in hot climates

In tropical countries glasshouses are ventilated to let hot air escape, because temperatures that are too high would denature the enzymes of photosynthesis and reduce yield.

15Yeast in dough

Enzymes from the yeast break starch in the flour into sugars. The yeast respires aerobically at first, then anaerobically once oxygen in the dough is used up, producing the CO₂ that is trapped as gas pockets.

16Yeast fermentation practical

Cover yeast and sugar solution with a layer of oil to keep oxygen out, then lead the gas into limewater or water and count bubbles in a fixed time at each water-bath temperature. Keep yeast mass, sugar concentration, volume and pH constant.

17Fermenter products

Industrial fermenters grow microorganisms on a large scale for products such as mycoprotein, yoghurt cultures, antibiotics (e.g. penicillin from a mould) and proteins made by genetically modified bacteria.

18Why farm fish

Overfishing has depleted many wild stocks. Farming gives a reliable protein supply, fish free of some pollutants found in wild fish, and control over growth, but it is costly and labour-intensive.

19Fish farm details

Different species are kept apart by nets or tanks to prevent interspecific predation; food rich in protein is given little and often so none is wasted and fish do not overeat; sexes can be kept apart so only chosen fish breed.

20Glasshouse biological control

Biological control works especially well in enclosed glasshouses: for example, parasitic wasps are released to control whitefly on tomato crops. Pesticides include insecticides, herbicides and fungicides, and insecticides may also kill useful pollinators such as bees.

Part 4 of 8Process & sequence

- Heat milk to about 90 °C to kill unwanted bacteria (pasteurise) and cool to about 40 °C.
- Add a starter culture of Lactobacillus bacteria.
- Incubate at about 40–46 °C for several hours.
- The bacteria respire anaerobically, converting lactose into lactic acid.
- Lactic acid lowers the pH, causing milk proteins to coagulate and thicken, and giving a sour taste.
- Cool to stop fermentation; add flavourings or fruit and package.
Investigating CO₂ production by yeastwrite these steps in this order; each step is a marking point.
1. Mix a fixed mass of yeast with a fixed volume and concentration of sugar solution in a boiling tube.
2. Place the tube in a water bath at the chosen temperature and leave to equilibrate.
3. Connect a delivery tube into an upturned measuring cylinder of water (or count bubbles).
4. Measure the volume of gas collected in a fixed time.
5. Repeat at different temperatures, keeping yeast mass and sugar concentration constant;
  - calculate rates and means.

Part 5 of 8Worked examples

60%

Tomato yield in open fields was 45 tonnes/ha. In a glasshouse with added CO₂ it was 72 tonnes/ha. Calculate the percentage increase.

Increase = 72 − 45 = 27 tonnes/ha. Percentage = 27 ÷ 45 × 100 = 60%.

Yeast in sugar solution produced 18.6 cm³ of CO₂ in 6 minutes at 25 °C and 42.0 cm³ in 6 minutes at 35 °C. Calculate each rate and explain the difference.

25 °C: 18.6 ÷ 6 = 3.1 cm³/min. 35 °C: 42.0 ÷ 6 = 7.0 cm³/min.
- At higher temperature molecules have more kinetic energy so there are more successful collisions between enzymes and substrate;
  - respiration is faster (below the optimum).

Loss of $25

A farmer spends $400 on CO₂ enrichment and gains 1.5 tonnes extra lettuce, sold at $250 per tonne. Is it worthwhile?

Extra income = 1.5 × 250 = $375. $375 < $400, so there is a loss of $25;
- CO₂ is probably no longer the limiting factor.

Part 6 of 8Summary

- Glasshouses control temperature, CO₂ and light to increase photosynthesis and yield.
- Fertilisers supply mineral ions such as nitrate for protein synthesis.
- Biological control is specific and leaves no residues but is slower.
- Yeast respires anaerobically to make CO₂ (bread) and ethanol (beer, wine).
- Lactobacillus converts lactose into lactic acid to make yoghurt.

Part 7 of 8Keep going

Food Production Edexcel O Level Biology: what to do next
These Food Production Edexcel O Level Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Food Production Edexcel O Level Biology understanding with the practice questions below, then reread any Food Production Edexcel O Level Biology point you were unsure about before moving on.
Food Production Edexcel O Level Biology: revision checklist
- Revise the main Food Production Edexcel O Level Biology definitions until you can write them from memory.
- Practise Food Production Edexcel O Level Biology exam questions under timed conditions.
- Link each Food Production Edexcel O Level Biology idea to a diagram or a worked example.
- List the common Food Production Edexcel O Level Biology mistakes and how to avoid them.
- Explain one Food Production Edexcel O Level Biology idea out loud as if teaching a friend.
- Summarise Food Production Edexcel O Level Biology in five key points on a revision card.
- Practice: Food Production Edexcel O Level Biology practice questions
- Previous lesson: 4.4 Human Influences on the Environment — O Level Notes
- Next lesson: 5.2 Selective Breeding — O Level Notes
- Course: Edexcel O Level Biology — all units
- Official syllabus: Pearson Edexcel International GCSEs

Part 8 of 8Compare: Compare

Pesticides vs biological control
Feature	Pesticides	Biological control
Speed of action	Fast	Slow
Specificity	Often kill non-target species	Usually specific to the pest
Persistence / bioaccumulation	May persist and accumulate in food chains	No chemical residues
Resistance	Pests can evolve resistance; repeated applications needed	Resistance rare; self-sustaining
Risk	Toxic residues on food	Control agent may itself become a pest

Conditions in a fermenter and why
Condition	Reason
Sterilised with steam (aseptic)	Prevent contamination by other microorganisms that compete or produce toxins
Nutrients added	Supply glucose (energy source) and amino acids/nitrogen for growth
Optimum temperature (water jacket)	Microbial respiration releases heat; enzymes denature if too hot
Optimum pH (probe, add acid/alkali)	Enzymes work best at optimum pH
Oxygen/sterile air (if aerobic)	For aerobic respiration
Stirring paddles	Mix nutrients, oxygen and heat evenly

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Glasshouse (greenhouse) · Limiting factor · Crop yield · Fertiliser · Pesticide · Biological control · Fermentation (in food production) · Lactobacillus · Fermenter · Aseptic precautions · Fish farming · Intraspecific predation

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1

Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).

2Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

3Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

4

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

5Avoid vague words

“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.

6

Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.

Words to avoid

1

CO₂ is added to help plants breatheextra CO₂ increases the rate of photosynthesis if CO₂ is the limiting factor

2

fertiliser is plant foodfertiliser supplies mineral ions such as nitrate for making proteins

3

yeast makes bread rise by producing oxygenyeast produces carbon dioxide by anaerobic respiration, which makes dough rise

4

the fermenter is heated to keep bacteria alivethe water jacket removes heat released by respiration to maintain the optimum temperature for enzymes

5

biological control kills all the pestsbiological control reduces the pest population to an acceptable level