By the end of this lesson
- Set up and use a potometer safely.
- Plan a fair test of an environmental factor.
- Evaluate water uptake as a transpiration proxy.
BIG QUESTION
How can a moving air bubble reveal the rate at which a shoot loses water?
A potometer measures water uptake, which usually approximates transpiration when growth and storage changes are small.
Method: build an unbroken water pathway
Cut a healthy leafy shoot under water with a sharp blade so air is not drawn into xylem. Assemble the shoot and capillary completely full of water, then seal every joint.
Dry the leaf surfaces, introduce one air bubble and use a reservoir to return it to a start position.
Variables, control variables and data handling
Record bubble distance in a fixed time, or time to travel a fixed distance. Convert distance to volume if capillary cross-sectional area is known: volume = distance × area.
Change only one variable—such as lamp distance, fan speed, humidity or temperature—and allow acclimation. Keep leaf area, time, apparatus and other conditions constant as control variableA variable kept the same throughout an investigation so that it cannot affect the results (the dependent variable); it is not the same as a control group.. Repeat and calculate a mean, identify anomalous readings and plot mean rate of water uptake against the factor changed.
Interpretation and limitations
A potometer directly measures water uptake, not transpiration. Some absorbed water is used in photosynthesis, growth or maintaining turgor, and stored water may be lost.
Limitations: leaks, changing temperature, unequal leaf area and xylem air locks produce error. Measuring leaf area and reporting uptake per unit area improves comparison.
Safety and quality
Use a tile and cut away from fingers; handle glass capillary tubing carefully and wear eye protection if your laboratory procedure requires it.
Keep electrical lamps or fans away from water and use low-voltage equipment. Do not place the shoot so close to a lamp that temperature becomes an uncontrolled variable.
Graph skills: SLAPUK and choosing a scalePotometer rates are small decimals: choose a decimal scale.
Variables in a wind-speed investigation
A fair test isolates one environmental change.
| Variable type | Example | How managed |
|---|---|---|
| Independent | Fan speed or distance. | Set measured levels. |
| Dependent | Bubble distance per minute. | Use the same capillary scale and timer. |
| Control variable | Leaf area. | Use the same shoot throughout or normalise by area. |
| Control variable | Temperature/light/humidity. | Monitor and keep constant. |
| Reliability | Repeated readings. | Reset bubble and calculate a mean. |
Language that earns precision
- potometer
- Apparatus measuring water uptake by a cut shoot.
- proxy
- Indirect measure used to estimate another quantity.
- capillary tube
- Very narrow tube in which small volume changes are visible.
- acclimation
- Time allowed for a plant to adjust to new conditions before measurement.
Common misconceptions
Claim: A potometer directly measures transpiration.
Correction: It measures water uptake; this is an approximation to water loss.
Claim: A faster bubble means air is entering the plant.
Correction: The bubble marks water displacement in the capillary.
Claim: Moving a lamp changes only light intensity.
Correction: It may change leaf temperature too, so temperature must be monitored or controlled.
Match the answer to the command word
State independent, dependent and control variables plus repeats.
Lead with “measures water uptake, not transpiration directly”.
Use rate = distance ÷ time or volume ÷ time and include units.
Recall, apply, then justify
Describe how to obtain reliable potometer readings at three wind speeds.
Hint
Set up, vary, keep control variables constant, repeat.
Suggested marking points
- Cut the shoot under water and assemble a water-filled airtight apparatus.
- Introduce and reset a bubble to a start point.
- Set three measured fan speeds/distances.
- Measure bubble movement over the same time at each setting.
- Keep leaf area, light, temperature and humidity controlled.
- Repeat after acclimation and calculate means.
Explain two limitations of a potometer result.
Hint
Proxy and apparatus error.
Suggested marking points
- It measures water uptake rather than transpiration directly.
- Some water is used/stored, so uptake may not equal loss.
- Leaks or air locks alter bubble movement.
- Unequal leaf area or changing conditions make treatments incomparable.
Design a method to compare two species fairly using a potometer.
Planning hint
Normalise for leaf area and reduce biological variation.
What a strong design includes
- Use healthy shoots of similar developmental stage.
- Cut under water and test airtight apparatus.
- Expose both species to the same measured conditions.
- Allow the same acclimation time.
- Measure uptake volume per unit time.
- Measure total leaf area and calculate uptake per unit leaf area.
- Use several shoots of each species and calculate means.
- Report variation and avoid claiming water uptake equals transpiration exactly.