# 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.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 5.1 Microbial growth and culture · Explain · 5 marks

> A liquid growth medium contains carbohydrates, proteins, vitamins and minerals. The flask is sealed with a cotton wool bung.

## Mark scheme (5 marks)

1. The growth medium contains carbohydrates/proteins/vitamins/minerals which provide nutrients for bacterial growth
2. The nutrients/carbohydrates/proteins in the growth medium are used up / become limiting
3. Toxic waste products produced by the bacteria accumulate in the medium / inhibit bacterial growth
4. The rate of bacterial death equals the rate of reproduction / population reaches a maximum (carrying capacity)
5. Oxygen/space in the sealed flask is limited, restricting bacterial growth

## Key terms

- [growth medium](https://www.gradenine.co.uk/glossary/growth-medium)
- [bacteria](https://www.gradenine.co.uk/glossary/bacteria)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-scientist-is-investigating-the-growth-d5e44cfd) · Published by Druglandscape Ltd.