When a potometer bubble moves for different lengths of time in two trials, divide each distance by its own time. In the same uniform capillary tube, a faster-moving bubble indicates faster water uptake.
The 2027 SEC G3 Biology K325 syllabus includes investigating how air movement affects transpiration. A bubble potometer measures water uptake by a shoot, which can be used to estimate transpiration under suitable conditions.
- Record the distance and time for each trial.
- Convert to the same units before comparing rates.
- Check that the same uniform tube was used.
- Separate what was measured from what was inferred.
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Identify what the apparatus measures
As the shoot takes up water, the air bubble moves. In a tube with a uniform bore, equal lengths represent equal volumes. Divide distance by time to calculate bubble speed. Multiply that speed by the tube’s internal cross-sectional area if you need volume per unit time.
The shoot can retain or use some of the water it takes up. Describe the measurement as water uptake first. Then explain why a controlled comparison of uptake provides evidence about transpiration.
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Worked example | A longer distance in still air
In moving air, the bubble moves 24 mm in four minutes. In still air, it moves 30 mm in six minutes. The moving-air rate is 24 ÷ 4 = 6 mm per minute. The still-air rate is 30 ÷ 6 = 5 mm per minute. The still-air trial gives the longer distance because it lasts longer.
The results indicate faster water uptake in moving air. Moving air can carry water vapour away from the leaf surface, maintaining a steeper water vapour concentration gradient between the leaf air spaces and the surrounding air. This can increase the rate at which water vapour diffuses out.
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Check the conditions before drawing a conclusion
Use the same shoot and apparatus where practical. Keep light, temperature and the humidity of the surrounding air as similar as possible. Check for leaks, allow time for the shoot to settle after changing conditions and repeat the measurements.
Try another reading. A bubble that travels 36 mm in six minutes moves at 6 mm per minute. That matches the original moving-air rate, even though the bubble travelled 12 mm farther.
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Frequently asked questions
Can I compare bubble speeds in tubes of different widths?
Convert each speed to volume per unit time using that tube’s internal cross-sectional area. Equal speeds in different tubes can represent different uptake rates.
How do I support a conclusion?
Give both rates and identify which condition had the higher rate. If you infer faster transpiration, identify the measured uptake as your evidence.