Plant organ system: Students should be able to understand and use simple compound measures such as the rate of transpiration.

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Q: Is this Plant organ system: Students should be able to understand and use simple compound measures such as the rate of transpiration. guide aligned to the AQA specification?

Yes. This guide is linked to the Plant organ system: Students should be able to understand and use simple compound measures such as the rate of transpiration. topic in the AQA GCSE specification.

A compound measure combines two different quantities, so its unit is a combination of two units.

Study notes

From the checked teaching notes for this specification point.

The idea

A compound measure combines two different quantities, so its unit is a combination of two units. Rate of transpiration is a compound measure because it compares a volume of water lost with a time taken. It is calculated as rate = volume of water lost ÷ time taken, giving units such as cm³ per minute or cm³ min⁻¹. A potometer can estimate water uptake by a leafy shoot, and the distance moved by an air bubble along a capillary tube can be converted into a volume using the tube's cross-sectional area. They should also be able to compare rates, convert units such as minutes to seconds, and use a rate to find a volume or a time.

What earns the marks

A compound measure combines two quantities, so its unit combines two units, for example cm³ per minute.
Rate of transpiration = volume of water lost ÷ time taken.
A potometer estimates transpiration by measuring water uptake by a leafy shoot, often from the movement of an air bubble.
Bubble distance can be converted to volume using the cross-sectional area of the capillary tube.
A larger calculated rate means water is being lost more quickly.
Rearranging the relationship allows a volume or a time to be found from a known rate.

Common mistakes and how to put them right

Dividing time by volume.
Put it right by dividing volume of water lost by time taken.
Writing the unit as cm³ when a rate is required.
Put it right by including the time unit, for example cm³ per minute.
Mixing minutes and seconds in one calculation.
Put it right by converting all times to the same unit before dividing.

Exam tips

Write the equation, substitute the values with units, then give the answer with the correct compound unit.
Check whether the question asks for a rate, a volume or a time, and rearrange the equation before substituting.
Use the gradient of a volume against time graph to find the rate, and compare gradients to compare rates.

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.

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A plant loses 30 cm³ of water in 5 minutes. What is the rate of transpiration in cm³ per minute?

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