IGCSE · Practice questions · Fermenters

Control the conditions, control the yield.

Six original Cambridge-style questions on fermenters: products made by bacteria and fungi, the parts of a fermenter and why each condition must be controlled, with growth curve data.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Part, condition, reason.

Products: insulin (GM bacteria), penicillin (Penicillium), mycoprotein (Fusarium).
Five conditions: temperature, pH, oxygen, nutrient supply, waste products.
Always link each condition to enzymes, respiration or growth in your answer.
Sterile conditions stop contamination by other microorganisms.

Recall

[3 marks]

Extended. Name the organism used to make each product in a fermenter: (a) insulin, (b) penicillin, (c) mycoprotein.

(a) Bacteria that have been genetically modified with the human insulin gene. ✓
(b) The fungus Penicillium. ✓
(c) The fungus Fusarium. ✓

Explain

[4 marks]

Extended. Explain why the temperature inside a fermenter must be controlled and how this is done.

The microorganisms respire and release heat, so the temperature rises. ✓
If it rises above the optimum, enzymes are denatured, so growth and product formation fall. ✓
If it is too low, enzymes work slowly and growth is slow. ✓
A water jacket around the tank carries cold water that removes heat; a temperature probe monitors it. ✓

Explain

[3 marks]

Extended. Explain why sterile air is bubbled into a fermenter used to grow Fusarium.

The air supplies oxygen for aerobic respiration. ✓
Aerobic respiration releases more energy, so the fungus grows faster and gives a higher yield of mycoprotein. ✓
The air is sterile so that no other microorganisms contaminate the culture or compete with the fungus. ✓

Data

[4 marks]

The table shows the mass of microorganisms in two fermenters. In fermenter B the pH was not controlled.

Time / hours	0	8	16	24	32
Fermenter A / g dm⁻³	1	5	20	34	36
Fermenter B / g dm⁻³	1	4	11	15	16

(a) Calculate the difference in mass between A and B at 32 hours. (b) Explain the difference.

(a) 36 − 16 = 20 g dm⁻³. ✓
(b) In B, waste products (such as carbon dioxide or acids) change the pH away from the optimum. ✓
The enzymes of the microorganisms work more slowly (or are denatured) away from their optimum pH. ✓
So respiration and growth are slower and the final mass is lower; in A acid or alkali is added to keep the pH at the optimum. ✓

Analysis

[3 marks]

Extended. Suggest why growth in a fermenter eventually stops even when temperature and pH are controlled, if no fresh nutrients are added and no waste is removed.

The nutrients (such as sugars) are used up, so there is no more substrate for respiration and growth. ✓
Toxic waste products build up and can kill the microorganisms. ✓
So the population stops increasing and may decline. ✓

Recall

[3 marks]

Extended. Describe how fermenters can be used for the large-scale production of a useful product.

The fermenter is sterilised and filled with a sterile nutrient solution, then a pure culture of the bacterium or fungus is added. ✓
Conditions are kept at the optimum: temperature (water jacket), pH (probe, acid or alkali), oxygen (sterile air), nutrients (inlet), and waste gases removed; the contents are stirred. ✓
After growth, the culture is drained off and the product (for example insulin, penicillin or mycoprotein) is extracted and purified. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.