Transpiration Virtual Lab: Light, Humidity, Wind and Water
Updated 2026-10-06
This transpiration virtual lab matches the AP Biology transpiration investigation, and it covers the GCSE practical work on the factors that affect transpiration rate. Students set up a control, then change one environmental factor at a time: wind, air humidity, light and soil water. They record the transpiration rate and stomatal opening and explain each result with stomata and the water-vapor gradient. One period gives four data series, a percentage-change calculation and a clear link between water loss and CO₂ uptake.
Curriculum links
- AP Biology Investigation 11, Transpiration. It supports Unit 2 (Topic 2.8, tonicity and osmoregulation: water moves down a water-potential gradient) and the science practices of experimental design and data analysis.
- AQA GCSE Biology 4.2.3.2 (plant organ system): the effect of humidity, air flow and light intensity on the rate of transpiration, and the role of stomata and guard cells.
- NGSS HS-LS1-3: feedback mechanisms maintain homeostasis. Here, stomata close when the soil dries.
Simulic is not affiliated with or endorsed by the College Board or AQA.
Before the lab (5 min)
Ask students to commit to a prediction, on paper or as question 1 of the class link:
"A plant sits in bright light with moist soil. A fan raises the wind speed from 0 to 4 m/s. What happens to the transpiration rate?"
Many students expect no change ("wind doesn't open stomata") or a fall ("wind cools the leaf"). Don't correct them yet.
Method in the simulation
The readout under the plant shows the stomatal opening, the transpiration rate in g/dm²/hour and the water balance. The model has no randomness, so every group gets the same numbers.
- Set the control conditions: Light 100%, Humidity 40%, Wind 0 m/s, Soil 70%. Record the rate.
- Wind: change only Wind to 1, 2, 4, 6 and 8 m/s. Record the rate and the water balance each time. Return Wind to 0.
- Humidity: change only Humidity to 20, 60, 80 and 100%. Return it to 40%.
- Light: change only Light to 0 and 50%. Return it to 100%.
- Soil water: change only Soil to 45, 35, 25 and 5%. Record the stomatal opening as well.
- Plot each series as rate against the factor, with the control point on every graph.
| Factor changed | Setting | Stomatal opening (%) | Transpiration rate (g/dm²/hour) |
|---|---|---|---|
| None (control) | L 100, H 40, W 0, S 70 | ||
| Wind (m/s) | 1 / 2 / 4 / 6 / 8 | ||
| Humidity (%) | 20 / 60 / 80 / 100 | ||
| Light (%) | 0 / 50 | ||
| Soil (%) | 45 / 35 / 25 / 5 |
Expected results
All readings come from the simulation at the settings above. The control rate is 1.44 g/dm²/hour with the stomata 100% open.
| Wind (m/s) | 0 | 1 | 2 | 4 | 6 | 8 |
|---|---|---|---|---|---|---|
| Rate | 1.44 | 1.94 | 2.45 | 3.46 | 4.46 | 5.47 |
| Humidity (%) | 20 | 40 | 60 | 80 | 100 |
|---|---|---|---|---|---|
| Rate | 1.92 | 1.44 | 0.96 | 0.48 | 0.00 |
| Soil (%) | 70 | 45 | 35 | 25 | 5 |
|---|---|---|---|---|---|
| Stomatal opening (%) | 100 | 100 | 75 | 50 | 0 |
| Rate | 1.44 | 1.44 | 1.08 | 0.72 | 0.00 |
- Wind raises the rate in a straight line: 4 m/s gives 140% more than still air. From 2 m/s the water balance turns negative: the leaf loses water faster than the roots supply it.
- Humidity lowers the rate in a straight line, to zero in saturated air: there is no vapor gradient left.
- Light opens the stomata: 0% gives 0.00, 50% gives 0.72, half the control.
- Soil water has no effect until it falls below 45%. Then the stomata close, even in full light.
Questions for students
- (Prediction, asked again after the lab) What happens to the transpiration rate when the wind rises from 0 to 4 m/s?
- When you test wind speed, which conditions must stay the same?
- At the control conditions, what is the transpiration rate?
- By what percentage does the rate increase when the wind goes from 0 to 4 m/s?
- Why does lowering the soil water to 25% halve the rate in full light, and what does this cost the plant?
Answers for teachers: (1) It rises, from 1.44 to 3.46 g/dm²/hour. (2) Light, humidity and soil moisture. (3) 1.44 g/dm²/hour. (4) About 140% (accept 137–143%). (5) Dry soil triggers ABA, the guard cells lose water and the stomata half close, so less vapor escapes. Less CO₂ enters too, so photosynthesis slows.
Common misconceptions
- "Transpiration is the plant sweating to cool down." Cooling is a side effect. Water evaporates inside the leaf and diffuses out through open stomata, down a vapor gradient.
- "Wind dries the soil, so it lowers transpiration." Wind sweeps away the moist air layer next to the leaf, which steepens the gradient and raises the rate.
- "Stomata only respond to light." Water shortage overrides light: at 5% soil water the stomata are shut at full light.
Extension
- Press Blazing noon (light 100%, humidity 30%, wind 2.5 m/s, soil 30%). The stomata are only 63% open, yet the rate is 1.97 and the water balance is −1.07. Ask why plants wilt at midday even though they close their stomata partly.
- The AP lab reports water loss per leaf area. The simulation already gives g per dm² per hour. Ask students to convert the control rate to g/m²/hour (144) and to explain why rates are compared per unit leaf area.
FAQ
Are the transpiration rates realistic?
They are teaching values chosen so the trends are easy to see. The direction and shape of each effect match real plants. Don't compare them with potometer readings.
Can students change temperature?
No. The simulation has sliders for light, humidity, wind and soil moisture only. Discuss temperature as a factor that raises evaporation and diffusion, or test it in a wet lab.
Can students measure stomatal density?
Not in this simulation. It shows one stoma in close-up, not a leaf peel. Pair it with a microscope activity if your course needs stomatal counts.
Related simulations and guides
Photosynthesis – counting oxygen bubbles in pondweed
Diffusion and Osmosis – Membrane Model and Potato Strip Experiment
For more biology activities, see interactive biology lesson ideas. To run the prediction as a full cycle, see predict, observe, explain with simulations.