A student wants to grow bacteria on agar plates to investigate the effect of temperature on the growth of microorganisms. Before the investigation begins, the student sterilises all equipment using an autoclave. Explain why each step in preparing and using the growth medium must be carried out under aseptic techniques, and describe what the growth medium must contain to allow successful culture of the bacteria.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is preparing agar plates to culture bacteria at different temperatures. The student sterilises all equipment using an autoclave before beginning the investigation.
Model answer (5 marks)
Aseptic techniques are used to minimise contamination of the culture.
If the agar plate or inoculating loop is contaminated, other microorganisms may grow and compete with the test bacteria, giving unreliable or misleading results.
The autoclave sterilises equipment by exposing it to high temperature and pressure, which kills all microorganisms and destroys any contaminants that could otherwise grow on the medium.
The growth medium must contain:
1. Carbohydrates – an energy and carbon source for bacterial metabolism.
2. Proteins – provide nitrogen, amino acids and peptides.
3. Vitamins – essential growth factors.
4. Minerals – required for enzymatic co‑factors and cellular functions.
If the agar plate or inoculating loop is contaminated, other microorganisms may grow and compete with the test bacteria, giving unreliable or misleading results.
The autoclave sterilises equipment by exposing it to high temperature and pressure, which kills all microorganisms and destroys any contaminants that could otherwise grow on the medium.
The growth medium must contain:
1. Carbohydrates – an energy and carbon source for bacterial metabolism.
2. Proteins – provide nitrogen, amino acids and peptides.
3. Vitamins – essential growth factors.
4. Minerals – required for enzymatic co‑factors and cellular functions.
Examiner tips
- Use the word ‘aseptic’ and explain contamination risk; mention autoclave temperature/pressure; list all four medium components; keep answer concise and use exam terminology.
Common mistakes
- Leaving out the mineral requirement; confusing the purpose of the autoclave with simple heat; not linking contamination to unreliable results.
Mark scheme (5 marks)
- Aseptic techniques are used to prevent/minimise contamination of the culture
- Contamination could introduce other microorganisms that would affect the results / make results unreliable
- The autoclave sterilises equipment using high temperatures (to kill microorganisms / to destroy contaminating microorganisms)
- The growth medium must contain carbohydrates (as an energy/carbon source) for bacterial growth
- The growth medium must also contain proteins AND vitamins AND minerals (all three required for 1 mark)
Key terms in this question
aseptic techniques · autoclave · growth medium · culture
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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