Question 1
Which gas, made by yeast, causes bread dough to rise?
- Oxygen
- Carbon dioxide
- Ethanol
- Nitrogen
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Answer: B (1)
Question 2
Describe the role of Lactobacillus in the production of yoghurt.
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- the bacteria respire / break down lactose (milk sugar) (1)
- to make lactic acid (1)
- which lowers the pH / makes the milk proteins clump so milk thickens (1)
- allow gives a sour taste
Question 3
A student investigated the effect of temperature on the rate of anaerobic respiration in yeast. She mixed yeast with sugar solution, added a layer of oil, and collected the gas in a syringe in a water bath. (a) Explain why a layer of oil was added. (b) State two variables she should control.
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- (a) oil stops oxygen entering the solution (1)
- so the yeast can only respire anaerobically (1)
- (b) any two from: volume of sugar solution / concentration of sugar / type of sugar / mass of yeast / time for collecting gas (1)(1)
- allow depth / volume of oil
- reject temperature (independent variable)
Question 4
In the investigation at 30 °C, 12 bubbles of gas were counted in 3 minutes. Calculate the mean number of bubbles produced per minute.
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- 12 ÷ 3 (1)
- 4 bubbles per minute (1)
Question 5
An industrial fermenter has a water jacket and stirring paddles. Explain the purpose of each of these features.
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- water jacket: cools the fermenter / keeps the temperature at the optimum (1)
- because respiration releases heat / heat would denature enzymes (1)
- paddles mix the contents so nutrients and oxygen are evenly distributed (1)
Question 6
Describe and explain the conditions needed in an industrial fermenter for the growth of micro-organisms.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | Describes and explains at least four conditions, with a clear reason for each linked to growth or enzyme activity, and accurate use of terms such as aseptic, optimum, oxygenation and agitation. |
| 2 | 3–4 | Describes at least three conditions with some explanation of why they are needed. |
| 1 | 1–2 | Names one or two conditions with little or no explanation. |
Indicative content
- aseptic precautions: sterilise equipment and nutrients to stop contamination by other microbes that compete or spoil the product
- nutrients such as glucose and nitrogen sources for growth
- optimum temperature, controlled with a cooling jacket, for fastest enzyme activity
- optimum pH, monitored with probes and controlled by adding acid or alkali, to avoid denaturing enzymes
- oxygenation: sterile air supplied for aerobic respiration, giving more energy for growth
- agitation: stirring so nutrients, oxygen and heat are evenly spread