Potometer Experiment for O-Level Biology: Transpiration Practical Guide

• Leak-test before timed runs: open the reservoir to push the meniscus along; if it slips while valves are closed, revisit joints.
• Keep the reservoir tap closed during measurements to prevent unwanted compensation flow.
• Stabilise temperature by waiting 2–3 minutes between changing environmental factors and starting a timed trial.

3 | Core investigations to rehearse

1. Baseline transpiration rate
   • Place the setup in ambient lab conditions (approx. 26 °C, 50 % relative humidity).
   • Record meniscus movement every 60 s for 6 minutes.
   • Convert to volume by multiplying distance by cross-sectional area; present both raw displacement (mm) and volume (cm³).
2. Light intensity sweep
   • Position a lamp at 20 cm, 40 cm, 60 cm.
   • Measure illuminance with a light meter or state the relative lux values provided in the paper.
   • Expect higher rates at closer distances because stomata open wider under stronger light.
3. Airflow control
   • Use a desk fan set to low/medium/high speeds or position a cardboard windbreak for the “still air” control.
   • Comment on how increased air movement maintains a steep diffusion gradient and accelerates transpiration.
4. Humidity adjustment
   • Enclose the shoot in a clear plastic bag with a damp tissue (high humidity) versus dry air (control).
   • Link reduced rate to smaller diffusion gradient when external air nears saturation.

Use a standard results table so PDO marks stay safe:

Fill in actual numbers during practice; include units in column headings to satisfy PDO descriptors.

4 | ACE commentary that examiners reward

• Explain trends: Connect higher light intensity to increased photosynthesis and stomatal opening; relate fan trials to faster removal of boundary-layer water vapour.
• Compare magnitudes: Quantify how many times faster the rate is under the lamp relative to ambient; avoid “faster/slower” without numbers.
• Evaluate reliability: Comment on whether the rate stabilised after the initial surge, note any bubbles that disrupted flow, and suggest repeating trials to compute a mean.
• Discuss limitations: Recognise that the potometer measures water uptake, not direct transpiration, and account for water used in photosynthesis.
• Propose improvements: Suggest using a data logger to capture continuous displacement, or a bubble potometer with a horizontal capillary to reduce gravity effects.

5 | Modification prompts to anticipate

• Hydrogencarbonate indicator follow-up: Suggest sealing leaves in indicator to link stomatal opening to CO₂ uptake; cite the indicator from SEAB’s reagent list.
• Leaf surface area normalisation: Recommend tracing leaves onto graph paper after trials to calculate rate per cm².
• Species comparison: Plan how you would keep stem diameter, leaf count, and shoot length comparable between species to make the comparison fair.
• Nighttime simulation: Propose placing the setup in darkness and predicting reduced rate due to stomatal closure, then describe how you would collect the data.

6 | Safety and logistics checklist

• Use sharp blades or scalpels with a cutting mat and follow your school’s lab safety procedures.
• Handle bench lamps carefully: specify distance to avoid heat damage, note that incandescent bulbs raise local temperature.
• Keep the potometer upright or supported by a retort stand to prevent the meniscus slipping due to gravity.
• Dispose of plant material responsibly; dry benches after trials to avoid slipping hazards.

Sources

1. SEAB, GCE O-Level Biology Syllabus (6093) 2026

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