Culturing microorganisms (biology only) — AQA GCSE Biology
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Culturing microorganisms (biology only) explained
A colony or a clear area around a colony is roughly circular, so its cross-sectional area is calculated using the formula for the area of a circle, A = πr².
Read the full explanation
First measure the diameter of the colony or clear zone with a ruler, then halve it to find the radius. Substitute the radius into A = πr², using π = 3.14 or the calculator value, and give the area in mm² or cm². For example, a colony of diameter 10 mm has radius 5 mm, so area = π × 5² = 78.5 mm². If comparing clear areas around colonies, calculate each area separately and compare the values to judge which microorganism is more affected by an antibiotic.
Your focus
- Measure the diameter of a colony or clear area and convert it to a radius.
- Calculate cross-sectional area using A = πr² with correct units.
- Compare calculated areas to draw conclusions about microbial growth or antibiotic effectiveness.
Culturing microorganisms (biology only) exam tips
Marking Points
- Measure the diameter of the colony or clear area, then divide by 2 to obtain the radius.
- Substitute the radius into the formula A = πr².
- Use an appropriate value of π, such as 3.14, and carry out the calculation accurately.
- Give the area with correct units, for example mm² or cm².
- When comparing clear areas, calculate each area and use the values to support a conclusion about effectiveness.
- Show the stages of the calculation so the method can be followed.
Examiner Tips
- 💡Write down the measured diameter and the calculated radius before substituting into the formula.
- 💡Show each step of the calculation, including the value of π used.
- 💡Check that the final answer has squared units, such as mm².
Common Mistakes
- Using the diameter instead of the radius in A = πr²; the correction is to halve the diameter first.
- Forgetting to square the radius; the correction is to calculate r² before multiplying by π.
- Giving the answer without units; the correction is to state the area in mm² or cm².