Factors Affecting the Rate of Transpiration (WJEC GCSE Biology)
Revision Note
Factors Affecting the Rate of Transpiration
The rate at which transpiration occurs is affected by several environmental factors
Environmental factors & transpiration table
Environmental factor | Effect on transpiration rate | Explanation |
|---|---|---|
Light intensity | As light intensity increases, the transpiration rate increases | At high light intensities the rate of photosynthesis increases, so stomata open to allow gas exchange; this allows more water to diffuse out of the stomata |
Temperature | As temperature increases, the transpiration rate increases | At high temperatures water vapour molecules have more kinetic energy, increasing the number of water molecules that diffuse out of the stomata per unit time |
Wind speed/air movement | As wind speed increases, the transpiration rate increases | Wind moves water vapour away from the leaf surface after it diffuses out of the stomata; this increases the water vapour concentration gradient between the inside and the outside of the leaf |
Humidity | As humidity increases, the transpiration rate decreases | Humid air contains a high concentration of water vapour, decreasing the water vapour concentration gradient between the inside and the outside of the leaf |
Specified Practical: Investigating Factors Affecting Transpiration
It is possible to investigate the effect of environmental conditions on the rate of transpiration using a piece of apparatus called a potometer
A bubble potometer measures the uptake of water by a plant shoot; this can be used as a measure of the volume of water lost in transpiration
This experiment assumes that water uptake and water loss by transpiration are equal
In reality, there will be a very small difference between the volume of water taken up and the volume of water lost in transpiration, as some water is used in photosynthesis; this means that potometers provide an indirect measure of transpiration rate
Potometer diagram
Bubble potometers can be used to measure the uptake of water by a plant shoot; this can be used to represent water loss in transpiration
Investigating the effect of environmental conditions on transpiration rate
Equipment
Potometer
Plant shoot
Basin or sink filled with water
Scalpel or sharp scissors
Vaseline
Paper towel
Beaker
Timer
Means of controlling an environmental factor
Method
Cut a shoot underwater using a diagonal cut
A diagonal cut creates a larger surface area for the uptake of water
Assemble the potometer underwater
Cutting and assembling underwater prevents air bubbles from entering the xylem where they could block the movement of water
Make sure that the apparatus is airtight, using Vaseline to seal any gaps
This prevents the entry of air into the system while the experiment is running
Dry the leaves of the shoot using a paper towel
Water on the leaves will block the stomata and affect transpiration
Allow a single air bubble to form inside the potometer and place the end of the tube into a beaker of water
The air bubble allows water movement in the tube to be observed
The beaker of water provides a supply of water to replace water that is lost during transpiration
Set up the environmental factor to be assessed (see below) and allow the plant shoot to adjust to the new environment for 5 minutes
Record the starting location of the air bubble
Leave the potometer for a set period of time, e.g. 15 minutes
Record the end location of the air bubble
Reset the bubble to its starting position by opening the tap below the reservoir
Repeat steps 7-10 twice more under the same environmental conditions before repeating the experiment under changed environmental conditions
Calculate the rate of transpiration:
Risk assessment
A hazard in an experiment is something that could potentially harm you
A risk is the harm that could occur as a result of the risk
A control measure is the action taken to reduce the chance of the hazard causing a problem
Risk assessment for transpiration practical table
Hazard | Risk | Control measure |
|---|---|---|
Sharp scalpel or scissors | Cuts to hands when cutting the plant shoot | Cut away from fingers |
Delicate glassware may break, resulting in sharp edges | Cuts to hands while assembling apparatus or clearing up breakages | Be gentle while assembling equipment |
Variables
The dependent variable in this investigation is the distance moved by the air bubble in a set time period
This can be used to calculate a rate of transpiration
There are a range of independent variables that could be tested for this investigation; some examples are given in the table below
The control variables depend on the independent variable being investigated:
Independent and control variables table
Possible environmental factor | Means of controlling environmental factor | Control variables |
|---|---|---|
Light intensity | Lamp placed at varying distances from potometer | Temperature, wind speed, humidity, leaf surface area |
Temperature | Heater with fine temperature control / an incubator | Light intensity, wind speed, humidity, leaf surface area |
Wind speed | Fan with a range of speed options | Temperature, light intensity, humidity, leaf surface area |
Humidity | Plastic bags placed over the shoot | Light intensity, temperature, wind speed, leaf surface area |
Leaf surface area | Repeating the experiment with different numbers of leaves | Light intensity, temperature, wind speed, humidity |
Results
We would expect to see the following results for the different independent variables:
Light intensity
As light intensity increases so does the rate of transpiration
Temperature
As temperature increases so does the rate of transpiration
Wind speed
As wind speed increases so does the rate of transpiration
Humidity
As humidity increases the rate of transpiration decreases
Leaf surface area
As leaf surface area increases so does the rate of transpiration
Examiner Tips and Tricks
You are expected to be able to evaluate practical methods such as this, so make sure that you are aware of the following:
How to keep any relevant control variables constant, e.g. maintaining a single light source while investigating the effects of temperature
How to ensure that your results are reliable, e.g. by carrying out repeats for each environmental condition
How to improve on the suggested method, e.g. always using fresh plant cuttings that are still transpiring
Hazards, risks, and corresponding control measures
Note that setting up potometers can be quite tricky, so it is possible that your teacher or lab technician may do it in advance; you still need to understand the purpose of the under-water set-up process even if you don't carry out this part of the practical yourself!
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