Measuring water uptake - potometers
Investigating transpiration
The uptake of water can be measured using a potometer. Under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration - this ignores the comparatively small amount of water used up in photosynthesis.
A simple potometer is a piece of capillary tubing to which a plant has been connected. The water uptake is measured by recording the time taken for a bubble in the tube to move a set distance.
Aim of the experiment
To find the rate of water uptake of a plant.
Image caption, The potometer is filled with water.
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Variables
- Independent variable – time.
- Dependent variable – the distance moved by the bubble along the capillary tube.
- Control variables – temperature, air flow or draughts, adequate supply of water.
Risks
Care must be taken when inserting the plant shoot into the rubber tubing attached to the capillary tube and with the scissors used to cut the plant shoot.
Example results
| Time (minutes) | Distance moved by bubble (mm) |
| 0 | 0 |
| 5 | 16 |
| 10 | 32 |
| 15 | 47 |
| 20 | 64 |
| 25 | 80 |
| 30 | 95 |
| Time (minutes) | 0 |
|---|---|
| Distance moved by bubble (mm) | 0 |
| Time (minutes) | 5 |
|---|---|
| Distance moved by bubble (mm) | 16 |
| Time (minutes) | 10 |
|---|---|
| Distance moved by bubble (mm) | 32 |
| Time (minutes) | 15 |
|---|---|
| Distance moved by bubble (mm) | 47 |
| Time (minutes) | 20 |
|---|---|
| Distance moved by bubble (mm) | 64 |
| Time (minutes) | 25 |
|---|---|
| Distance moved by bubble (mm) | 80 |
| Time (minutes) | 30 |
|---|---|
| Distance moved by bubble (mm) | 95 |
Analysis of results
Plot a line graph of the data. Make sure you:
- put the independent variable on the x-axis and the dependant variable on the y-axis
- construct regular scales for the axes
- label the axes appropriate
- plot each point accurately
- draw a line of best fit
The rate of water uptake is found using the In a graph, the gradient is the steepness of the line. The greater the gradient, the greater the rate of change. of the graph.
You need to calculate the gradient of the graph – and not take values from the table of results.
Choose any two points on the straight line:
- choose as wide an interval along the The line that runs along the bottom of a graph. as is possible
- try to make sure that these correspond with values on the y axis that you can read accurately
The most accurate results will be obtained if these points are as far apart as possible. Here, the two points on the line, labelled P and Q, are at either end of the line.
Calculating the gradient
P
Value of x = 0 minutes
Value of y = 0 millimetres
Q
Value of x = 30 minutes
Value of y = 96 millimetres
\(\text{Gradient of line} = \frac{\text{increase in y}}{\text{increase in x}}\)
\(= \frac{(96 - 0)~\text{mm}}{(30 - 0)~\text{min}} = 3.2~\text{mm/minute}\)
The distance moved by the bubble is 3.2 mm/minute.
The distance moved by the bubble is 3.2 mm/minute.
Note that rate is always a A compound measure is made up of two or more other measurements, eg rate of water loss is a compound measure because it is made up of water loss - for instance in grams - and time - for instance in minutes.. It is made up of two – or more – measurements. In this case, millimetres per minute
Question
When will the water uptake of a plant not equal the rate of transpiration?
When it is short of water, eg it has wilted.
More guides on this topic
- What happens during photosynthesis? - OCR 21st Century
- How do producers get the substances they need? - OCR 21st Century
- How are organisms in an ecosystem interdependent? - OCR 21st Century
- How are levels maintained within an ecosystem? - OCR 21st Century
- How are populations affected by conditions in an ecosystem?
- Sample exam questions - living together - OCR 21st Century