A transpiration data question gives numbers and expects a trend plus a reason. Describe what the numbers do first, then link the change to the factor that caused it.
This lesson belongs to transport in plants. If the route of water is unclear, start with explaining water movement through a plant.
What do you do with a transpiration table?
Work in three steps: read, calculate, explain. Each step earns marks separately.
- Read the headings and units before looking at the numbers.
- Calculate the rate if the question asks, using change divided by time.
- Explain the trend using the factor that changed.
Worked example: a potometer table
A student measures water uptake by a leafy shoot in four conditions. Each reading lasts 5 minutes.
| Condition | Bubble moved (mm) |
|---|---|
| Still air, shade | 10 |
| Still air, sunlight | 20 |
| Windy, sunlight | 35 |
| Windy, sunlight, leaves covered in jelly | 5 |
Rate in windy sunlight. 35 mm in 5 minutes = 7 mm per minute. Rate in still shade = 10 ÷ 5 = 2 mm per minute.
Describe. The rate rises from 2 to 4 to 7 mm per minute as light and wind are added.
Explain. Sunlight opens the stomata and raises temperature, so more water evaporates. Wind removes vapour near the stomata and makes the diffusion gradient steeper. The jelly blocks stomata, so the rate falls to 1 mm per minute.
Check: 20 ÷ 5 = 4 and 5 ÷ 5 = 1, so the four conditions give 2, 4, 7 and 1 mm per minute.
The mistake that loses marks
The common slip is to describe only: “the bubble moved more in the wind”. That is a description with no reason.
| Weak | Strong |
|---|---|
| The bubble moved more in the wind | Wind made the diffusion gradient steeper, so transpiration rose |
| The jelly made it lower | The jelly blocked the stomata, so less vapour escaped |
| 7 | 7 mm per minute |
Always include the unit and always finish with a reason.
Reading a graph
A graph follows the same steps. Name the axes, describe the trend with figures from the graph, and explain it with a factor. If the line levels off, say so and give a reason, such as stomata closing at midday.
Check yourself
A shoot takes up 24 mm of water in 6 minutes in dry air and 9 mm in 6 minutes in humid air. Calculate both rates and explain the difference.
Answer
Dry air: 24 ÷ 6 = 4 mm per minute. Humid air: 9 ÷ 6 = 1.5 mm per minute.
In dry air, the water vapour concentration outside the leaf is low, so the diffusion gradient is steeper. More water vapour leaves, so transpiration is faster.
What to study next
Move on to explaining translocation for the sugar journey. Then check your data skills with the transport in plants practice set or the biological data interpretation trainer.
If a teacher could turn your descriptions into explanations on fresh tables, see online one-to-one Biology tuition.