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Key concept
A potometer measures the rate of water uptake by a leafy shoot, which closely approximates transpiration rate. An air bubble in a capillary tube moves as the shoot absorbs water. The distance the bubble travels per unit time gives the uptake rate under controlled conditions.
Potometer questions appear on Papers 2, 4, 5 and 6. The most important distinction to remember is that a potometer measures water uptake, not transpiration directly: a small amount of absorbed water is used in photosynthesis rather than transpired.
How the potometer works
The leafy shoot is sealed into the potometer underwater to prevent air entering the xylem. A capillary tube with a scale extends horizontally from the apparatus. As the shoot loses water by transpiration, it draws water from the capillary tube, and the air bubble in the tube moves toward the shoot.
The distance the bubble moves in a set time gives the rate of water uptake. For example, if the bubble moves 30 mm in 5 minutes, the rate is 6 mm per minute. To reset, open the reservoir tap to push the bubble back to the start.
What the potometer actually measures
A potometer measures water uptake, not transpiration directly. However, since the vast majority of water absorbed by a plant is lost by transpiration (over 95%), water uptake is a very close estimate of transpiration rate. A small fraction of absorbed water is used in photosynthesis and to maintain turgor.
When writing up this practical, state clearly: "The potometer measures the rate of water uptake, which is used as an estimate of the rate of transpiration."
Variables and fair testing
To investigate the effect of one factor (for example, wind speed), change only that variable and keep all others constant. Use the same shoot for each reading if possible, or shoots of similar leaf area. Allow time for the shoot to acclimatise before recording.
Variable type
Example (investigating wind effect)
Independent
Wind speed (use a fan at different settings)
Dependent
Distance bubble moves per unit time
Controlled
Temperature, humidity, light intensity, leaf area, water temperature
Cut the shoot underwater to prevent air locks, and use Vaseline to seal all joints. Air entering the system gives unreliable results.
Try a focused question (4 marks)
A student uses a potometer to test the effect of a fan on water uptake. With the fan off, the bubble moves 12 mm in 5 minutes. With the fan on, it moves 28 mm in 5 minutes. Explain the difference.
Show the answer and marking guidance
The fan increases air movement (wind speed) around the leaves.
Wind removes the layer of humid air surrounding the leaf surface.
This steepens the water-vapour concentration gradient between the leaf interior and the air.
Water vapour diffuses out of stomata faster, increasing transpiration and therefore water uptake.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying the potometer measures transpiration directly rather than water uptake.
Not cutting the stem underwater, which introduces air bubbles that block flow.
Changing two variables at once (for example, adding a fan and a lamp).
Reporting bubble distance without dividing by time to get a rate.
Practical connection
Investigating the Rate of Transpiration
The rate of transpiration can be measured using a potometer: a device that tracks how fast water is taken up by a cut shoot. Factors such as wind, humidity, light, and temperature can be tested.