AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Students should be able to calculate cross-sectional areas of colonies or…
Full specification wording
Culturing microorganisms (biology only): Students should be able to calculate cross-sectional areas of colonies or clear areas around colonies using πr².
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.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 6 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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. 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.
What earns the marks
- 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.
Common mistakes and how to put them right
Using the diameter instead of the radius in A = πr².
Put it right: Halve the diameter first.
Forgetting to square the radius.
Put it right: Calculate r² before multiplying by π.
Giving the answer without units.
Put it right: State the area in mm² or cm².
Exam 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.
Try the lesson
The start of the Culturing microorganisms (biology only): Students should be able to calculate cross-sectional areas of colonies or clear areas around colonies using πr². lesson, made from checked teaching notes for this specification point.
Have a go first
A circular bacterial colony has a radius of 3 mm. Using the formula πr², what is the cross-sectional area of the colony? Use π = 3.14.
Listen
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. 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. A strong answer earns marks for these 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. A common slip: Using the diameter instead of the radius in A = πr². Instead, halve the diameter first. Exam tip: Write down the measured diameter and the calculated radius before substituting into the formula.
See it
Put it in order
Put these steps in the order they happen.
- 1. Compare the values to judge which microorganism is more affected by an antibiotic
- 2. To compare clear areas, calculate each area separately
- 3. Substitute the radius into A = πr²
- 4. Give the area with correct units, e.g. mm² or cm²
- 5. Measure the diameter of the colony or clear area with a ruler
- 6. Halve the diameter to find the radius
- 7. Square the radius, then multiply by π (e.g. 3.14)
Key words
Say what each word means before you turn the card.
Circular area calculation
Try answering aloud or in your head before turning the card.
Measurement and units
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Colony growth
Try answering aloud or in your head before turning the card.
Antibiotic clear zones
Try answering aloud or in your head before turning the card.
See every definition
- Circular area calculation
- 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.
- Measurement and units
- Measure the diameter with a ruler, halve it to find the radius, and give the area in mm² or cm².
- Colony growth
- The cross-sectional area of a colony is calculated using A = πr², with the radius found by halving the measured diameter.
- Antibiotic clear zones
- When comparing clear areas around colonies, calculate each area separately and compare the values to judge which microorganism is more affected by an antibiotic.
Label the diagram
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1Box 1
2Box 2
3Box 3
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