The Potometer and Measuring Rate of Transpiration
Joe Wolfensohn
Teacher
Recall Questions
This topic requires prior knowledge of transpiration and the transpiration stream.
How are xylem vessels adapted for transporting water?
They are hollow, have no end walls, and are strengthened with lignin for support.
What is the function of stomata in a leaf?
They allow gas exchange and control water loss by opening and closing.
How does increasing light intensity affect the rate of transpiration?
More light means more photosynthesis so the stomata open to allow in CO₂. This means there is more water loss and increased transpiration.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains the potometer and measuring rate of transpiration, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
The Potometer and Measuring Transpiration
What is a Potometer?
A potometer is a piece of apparatus used to estimate the rate of transpiration (water loss) from a plant.
Transpiration is the evaporation of water from the surface of leaves, through the stomata.
Diagram showing the set up of a potometer
Setting Up a Potometer:
- A leafy shoot is cut underwater to prevent air entering the xylem.
- The shoot is inserted into a water-filled potometer with a capillary tube.
- An air bubble is introduced into the capillary tube.
- As the shoot takes up water the air bubble in the capillary tube will move towards the plant.
- The distance the bubble moves is measured over time.
Independent variables you could investigate
|
Independent Variable |
Description |
|
Light intensity |
Vary distance of a lamp from the plant. |
|
Air movement (wind) |
Use a fan at different speeds / distances. |
|
Temperature |
Use a heater or carry out in different environments. |
|
Humidity |
Spray water in a bag around the plant to increase humidity. |
Dependent Variable
|
Dependent Variable |
How it’s measured |
|
Rate of transpiration |
Measure the distance an air bubble travels in a set time, then calculate the rate (e.g. mm per minute). |
Control Variables
These are the factors that must be kept constant to ensure valid results.
|
Control Variable |
How to keep it constant |
|
Type of plant |
Use the same species and size of shoot. If changed, this would affect the number of stomata. |
|
Leaf surface area |
Use the same number and size of leaves. If changed, this would affect the number of stomata. |
|
Temperature |
Unless it’s the independent variable, keep in a constant room temperature. |
|
Light intensity |
Unless it’s the independent variable, keep the lamp at the same distance from the shoot. |
|
Humidity |
Unless it’s the independent variable, keep consistent by working in the same environment. |
|
Time measured |
Measure for the same duration each time (e.g. 5 minutes). |
Key Terms
- Transpiration – the evaporation of water from a plant’s leaves via the stomata.
- Potometer – a device used to estimate the rate of transpiration from a plant.
No answer provided.
Exam Tip
- Make sure you can describe how to change each of the possible factors and all the variables that need to be controlled.
No answer provided.
Practice Questions
Two different plants, A and B were grown in the same conditions
- Which plant has a higher rate of transpiration? [1]
- Suggest why plant A lost more water in the first 2 hours [1]
- After 2 hours the plants were moved to a different room. Suggest why the rate of transpiration increased. [2]
Model Answer:
- A
- Larger plant / different species / more leaves / larger leaves
- Different conditions, such as higher temperature, higher wind speed, higher light intensity or lower humidity will increase the rate of transpiration.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!