Potometer (AQA AS Biology): Revision Note
Exam code: 7401
Investigating transpiration rate
The rate at which plants lose water by transpiration is affected by factors in the environment, such as:
air movement, or wind speed
humidity
light intensity
temperature
Environmental condition | Effect on transpiration rate | Explanation |
|---|---|---|
High wind speed | Increased | Wind moves water vapour away from the leaf surface after it diffuses out of the stomata; this increases the water potential gradient between the leaf and the air, increasing diffusion of water vapour |
High humidity | Decreased | The water potential gradient between the leaf and the air is reduced, slowing diffusion of water vapour |
High light intensity | Increased | The rate of photosynthesis increases, so stomata open to allow gas exchange; this allows more water to diffuse out of the stomata |
High temperature | Increased | Particles have more kinetic energy so water molecules evaporate from the mesophyll and diffuse out of the leaf more quickly |
Investigating the effect of an environmental variable on the rate of transpiration
A potometer can be used to investigate the effect of environmental factors on the rate of transpiration
Apparatus
Plant shoot
Basin of water
Scalpel/scissors
Paper towels
Potometer
Petroleum jelly
Beaker
Stopwatch
Means of controlling an environmental factor
Method
Cut a plant 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 potometer apparatus is airtight, using petroleum jelly 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:
Environmental factors can be investigated as follows:
airflow: set up a fan or hairdryer with different speeds
humidity: spray water in a plastic bag and wrap around the plant
light intensity: change the distance of a light source from the plant
temperature: carry out the experiment in a cold room and a warm room
Examiner Tips and Tricks
Note that, while potometers are used to measure transpiration rate, they don't actually truly measure transpiration, but instead provide a measure of the rate at which a plant shoot takes up water. Most of this water will then be lost in transpiration, but some may enter the cells and be used in photosynthesis.
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