A scientist is investigating the growth of bacteria in a liquid growth medium. The bacteria are grown in a flask at 37 °C. After 24 hours, the scientist notices that the number of bacteria has stopped increasing. Explain why the growth medium supports bacterial growth and suggest two reasons why the number of bacteria may have stopped increasing after 24 hours.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A liquid growth medium contains carbohydrates, proteins, vitamins and minerals. The flask is sealed with a cotton wool bung.
Model answer (5 marks)
The medium supplies carbohydrates, proteins, vitamins and minerals, which are the essential nutrients bacteria need for energy, macromolecule synthesis and growth.
After 24 h the growth stops because:
1. The nutrients (carbohydrates/proteins) are depleted or become limiting, so cells can no longer reproduce.
2. Accumulation of toxic waste products (e.g. acids, ammonia) inhibits further growth, and the death rate equals the birth rate, giving a carrying capacity.
(If oxygen or space were limiting, the same effect would occur.)
After 24 h the growth stops because:
1. The nutrients (carbohydrates/proteins) are depleted or become limiting, so cells can no longer reproduce.
2. Accumulation of toxic waste products (e.g. acids, ammonia) inhibits further growth, and the death rate equals the birth rate, giving a carrying capacity.
(If oxygen or space were limiting, the same effect would occur.)
Examiner tips
- Use the exact terms from the mark scheme – ‘nutrients’, ‘depleted’, ‘toxic waste’, ‘carrying capacity’.
- Show the two distinct reasons, not just one combined explanation.
- Mention the sealed flask only if you can link it to oxygen/space limitation.
Common mistakes
- Writing that the medium ‘does not contain nutrients’ – the medium actually does. Failing to give two separate reasons for the growth stop. Confusing nutrient depletion with lack of oxygen without justification.
Mark scheme (5 marks)
- The growth medium contains carbohydrates/proteins/vitamins/minerals which provide nutrients for bacterial growth
- The nutrients/carbohydrates/proteins in the growth medium are used up / become limiting
- Toxic waste products produced by the bacteria accumulate in the medium / inhibit bacterial growth
- The rate of bacterial death equals the rate of reproduction / population reaches a maximum (carrying capacity)
- Oxygen/space in the sealed flask is limited, restricting bacterial growth
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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