Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
5.19B Calculate cross-sectional areas of bacterial cultures and clear agar…
Full specification wording
Topic 5 – Health, disease and the development of medicines: 5.19B Calculate cross-sectional areas of bacterial cultures and clear agar jelly using π r 2
This statement asks you to calculate cross-sectional areas of circular bacterial cultures and of clear agar jelly (the inhibition zone) using the formula area = πr².
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 17 of 21
Study notes
From the checked teaching notes for this specification point.
The idea
This statement asks you to calculate cross-sectional areas of circular bacterial cultures and of clear agar jelly (the inhibition zone) using the formula area = πr². First find the radius: if you are given a diameter, halve it. Then square the radius and multiply by π (about 3.14). Keep units consistent, for example convert a diameter in millimetres to a radius in millimetres, then give the area in mm². For a clear zone, the area represents the region where microbial growth was inhibited. You may need to subtract the area of the disc from the total clear zone to find the jelly-only area. Show your working and round sensibly.
What earns the marks
- Halves a given diameter to obtain the radius before substituting into the formula.
- Squares the radius and multiplies by π to find the cross-sectional area.
- Uses consistent units and gives the area in squared units such as mm² or cm².
- Where required, subtracts the disc area from the total zone area to find the clear agar jelly area.
- Shows working and rounds the final answer appropriately.
Common mistakes and how to put them right
Using the diameter as the radius.
Put it right by halving the diameter first.
Forgetting to square the radius.
Put it right by applying r² before multiplying by π.
Mixing units, such as a radius in mm and an answer in cm².
Put it right by converting to one unit throughout.
Exam tips
- Write down the formula area = πr² before substituting values.
- Check whether the value given is a diameter or a radius.
- Include squared units in your final answer.
Try the lesson
The start of the Topic 5 – Health, disease and the development of medicines: 5.19B Calculate cross-sectional areas of bacterial cultures and clear agar jelly using π r 2 lesson, made from checked teaching notes for this specification point.
Have a go first
A circular bacterial culture has a radius of 3 cm. Using π = 3.14, calculate the cross-sectional area of the culture.
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