Rate calculations for transpirationSpec 6.13
A rate tells you how much something changes in a unit of time. For water uptake or transpiration you can use the distance moved by a bubble in a potometer, the volume of water lost, or the change in mass of a plant.
Rate of water uptake
In a potometer, an air bubble moved 31 mm, 28 mm and 34 mm in three repeats of 10 minutes each. Calculate the mean rate of water uptake in mm per minute.
- Mean distance = (31 + 28 + 34) ÷ 3 = 93 ÷ 3 = 31 mm.
- Rate = distance ÷ time = 31 ÷ 10 = 3.1 mm per minute.
Answer: 3.1 mm per minute
Percentage loss of mass
A leafy shoot had a mass of 25.0 g at the start of an investigation and 23.0 g at the end. Calculate the percentage loss of mass.
- Loss of mass = 25.0 − 23.0 = 2.0 g.
- Percentage loss = 2.0 ÷ 25.0 × 100 = 8%.
Answer: 8%
Quick check
What does a potometer measure?
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The rate of water uptake of a leafy shoot.
Why does moving air increase the rate of transpiration?
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It carries away water vapour, keeping a steep concentration gradient so water vapour diffuses out faster.
A bubble in a potometer moves 24 mm in 8 minutes. What is the rate?
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24 ÷ 8 = 3 mm per minute.
Why does a higher temperature increase the rate of water uptake?
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Water evaporates from the leaf cells faster and water vapour diffuses out faster, so more water is pulled up the xylem.