AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Students should be able to understand and use simple compound measures…
Full specification wording
Plant organ system: Students should be able to understand and use simple compound measures such as the rate of transpiration.
A compound measure combines two different quantities, for example distance per unit time or volume per unit time.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 3 of 13
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A compound measure combines two different quantities, for example distance per unit time or volume per unit time. The rate of transpiration is a compound measure because it compares an amount of water lost with the time taken. It can be expressed as volume of water lost per minute, for example cm³ min⁻¹, or as mass lost per minute, for example g min⁻¹, or as distance moved by an air bubble per minute, for example mm min⁻¹. To calculate it, divide the change in water lost or bubble distance by the time taken. To compare conditions fairly, use the same time interval or calculate a rate for each condition. A potometer measures water uptake, which is a close approximation of transpiration under the experimental conditions.
What earns the marks
- Identify the two component quantities in a compound measure: the amount of water lost, such as volume or mass, and the time taken.
- Calculate rate by dividing the change in water lost or bubble distance by the time taken, for example rate = volume ÷ time.
- Use correct compound units such as cm³ min⁻¹, g h⁻¹ or mm min⁻¹, and interpret what each unit means.
- Compare rates between conditions by calculating a value for each condition over the same time period, rather than comparing raw totals measured over different times.
- Recognise that a potometer measures water uptake by a shoot, which is used as an approximation of transpiration rate.
Common mistakes and how to put them right
Dividing time by volume instead of volume by time.
Put it right: Divide the amount of water lost by the time taken.
Comparing total water loss over different time periods without converting to a rate.
Put it right: Calculate each rate per unit time before comparing.
Writing units without the time component, such as cm³.
Put it right: Include the time unit, for example cm³ min⁻¹.
Exam tips
- Write the calculation in words first, then substitute the numbers, so the division is clear.
- Keep units consistent throughout a calculation and convert minutes to seconds or hours only when needed.
- Check whether the question asks for a rate or a total; a rate always includes a time unit.
Try the lesson
The start of the Plant organ system: Students should be able to understand and use simple compound measures such as the rate of transpiration. lesson, made from checked teaching notes for this specification point.
Have a go first
A potometer shows a leafy shoot taking up 30 mm³ of water in 5 minutes. What is the rate of transpiration in mm³ per minute?
Listen
A compound measure combines two different quantities, for example distance per unit time or volume per unit time. The rate of transpiration is a compound measure because it compares an amount of water lost with the time taken. It can be expressed as volume of water lost per minute, for example cm³ min⁻¹, or as mass lost per minute, for example g min⁻¹, or as distance moved by an air bubble per minute, for example mm min⁻¹. To calculate it, divide the change in water lost or bubble distance by the time taken. To compare conditions fairly, use the same time interval or calculate a rate for each condition. A potometer measures water uptake, which is a close approximation of transpiration under the experimental conditions. A strong answer earns marks for these points. Identify the two component quantities in a compound measure: the amount of water lost, such as volume or mass, and the time taken. Calculate rate by dividing the change in water lost or bubble distance by the time taken, for example rate = volume ÷ time. Use correct compound units such as cm³ min⁻¹, g h⁻¹ or mm min⁻¹, and interpret what each unit means. Compare rates between conditions by calculating a value for each condition over the same time period, rather than comparing raw totals measured over different times. A common slip: Dividing time by volume instead of volume by time. Instead, divide the amount of water lost by the time taken. Exam tip: Write the calculation in words first, then substitute the numbers, so the division is clear.
See it
Key words
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Compound measure
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Rate of transpiration
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Potometer
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Fair comparison
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See every definition
- Compound measure
- A measure that combines two different quantities, for example distance per unit time or volume per unit time.
- Rate of transpiration
- A compound measure because it compares an amount of water lost with the time taken.
- Potometer
- Measures water uptake by a shoot, which is used as an approximation of transpiration rate.
- Fair comparison
- Use the same time interval or calculate a rate for each condition rather than comparing raw totals measured over different times.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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