Rate calculations for transpirationSpec 6.13
In short
A rate of transpiration or water uptake is the change in a quantity divided by the time taken, for example the distance a bubble moves in a potometer per minute. Percentage change in mass equals change in mass divided by original mass, times 100. Calculate a mean from repeats first, and always give the unit, such as mm per minute.
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%
Always include the unit with a rate, for example mm per minute or cm³ per hour. Calculate a mean first if you are given repeats.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026