Transpiration and the potometer

Water moves up a plant in the transpiration stream. A potometer lets you measure how fast a plant takes up water in different conditions.

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The transpiration stream

1. Water enters root hair cells by osmosis (mineral ions enter by active transport).
2. It moves across the root into the xylem.
3. It travels up the xylem to the leaves.
4. It evaporates from cells inside the leaf and diffuses out through the stomata.
Losing water from the leaves pulls more water up the xylem.

Factors and why they work

FactorEffect on rateWhy
Light intensity upFasterStomata open wider for photosynthesis
Temperature upFasterWater molecules have more energy, so evaporation and diffusion are faster
Wind (air movement) upFasterWater vapour is blown away, keeping the concentration gradient steep
Humidity upSlowerMoist air reduces the concentration gradient

Using a potometer

Cut the shoot under water and assemble the potometer under water so no air enters the xylem. Seal joints with petroleum jelly and dry the leaves.
Measure how far an air bubble moves along the capillary tube in a set time.
rate = distance moved ÷ time
To find a volume: volume = π r2 × distance (r = radius of the capillary tube).
A potometer really measures water uptake; a small amount of water is used in photosynthesis, so this is only an estimate of transpiration.
Worked example

A bubble moves 30 mm in 5 minutes. The capillary tube has a radius of 0.5 mm. Calculate the rate of water uptake in mm3 per minute.

  1. Volume = π × 0.52 × 30 = 23.56 mm3
  2. Rate = 23.56 ÷ 5
  3. = 4.7 mm3 per minute (to 1 decimal place)

Answer: 4.7 mm3/min

Key idea

Water enters roots by osmosis, travels up the xylem and evaporates out of the stomata. Light, heat and wind speed it up; humidity slows it. A potometer measures the bubble distance in a set time: rate = distance ÷ time.

Key words
Transpiration
The loss of water vapour from the leaves of a plant, mainly through the stomata.
Osmosis
The net movement of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane.
Xylem
Dead, hollow plant tubes strengthened with lignin that carry water and mineral ions up from the roots.
Stomata
Pores, mostly on the underside of leaves, that let gases in and out. Guard cells open and close them.
Active transport
Movement of substances from a lower to a higher concentration, against the concentration gradient, using energy from respiration.
Capillary
The smallest blood vessel. Its wall is one cell thick so substances can be exchanged with tissues.

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Quick check

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Extra practice

2 more questions

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  1. A potometer bubble moves 36 mm in 4 minutes. Calculate the rate in mm per minute.

    Show the answer

    Answer: 9 mm/min

    36 ÷ 4 = 9 mm/min

  2. Why must a potometer be airtight?

    1. So air does not enter and the bubble moves only due to water uptake
    2. To keep the plant warm
    3. To stop photosynthesis
    4. To add carbon dioxide
    Show the answer

    Answer: So air does not enter and the bubble moves only due to water uptake

    Leaks give false readings.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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