Practical: Transpiration (SQA National 5 Biology): Revision Note

Investigating factors affecting the rate of transpiration

• We can investigate the effect of environmental conditions on the rate of transpiration using a piece of apparatus called a potometer

  • It is possible to investigate environmental factors such as:

    • wind speed

    • humidity

    • surface area

    • temperature

• There are two types of potometer:

  • a mass potometer measures the change in mass as a measure of the volume of water that has evaporated

  • a bubble potometer measures the uptake of water by a stem as a measure of the volume of water that is being lost by evaporation

Investigating the effect of wind speed on transpiration using a bubble potometer

Apparatus

• Leafy shoot

• Basin of water

• Scissors or scalpel

• Bubble potometer

• Petroleum jelly

• Fan

• Ruler

• Timer

Method

1. Cut a shoot underwater; the cut should be at an angle

   • Cutting under water prevents air from entering the xylem and causing a blockage

   • Cutting at an angle increases the surface area for water uptake

2. Set up the apparatus as shown in the diagram

   • Make sure it is airtight, using petroleum jelly to seal any gaps

   • Dry the leaves of the shoot as wet leaves will affect the results

3. Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water

4. Set up a fan 10cm from the leaf and turn it on

5. Allow the plant to adapt to the new environment for 5 minutes

6. Record the starting location of the air bubble

7. Leave for 30 minutes

8. Record the end location of the air bubble

9. Change the wind speed (low, medium or high)

10. Reset the bubble by opening the tap below the reservoir

11. Repeat the experiment

12. Calculate the rate of transpiration by dividing the distance the bubble travels by the time period

• Other environmental factors can be investigated using the same set-up:

  • Humidity: spray water in a plastic bag and wrap around the plant

  • Temperature: temperature of room (cold room or warm room)

Expected results

• As wind speed increases, the rate of transpiration should also increase

  • This will be shown by the bubble moving a greater distance in the 30 minute time period when the fan speed is increased from lower to higher speed

• Transpiration rate increases with wind speed because moving air removes the humid boundary layer of water vapour around the leaf surface

• This helps to maintain a steep diffusion gradient between the moist air inside the leaf and the drier air outside

• A steeper concentration gradient results in increased diffusion of water vapour out of the leaf through the stomata, therefore increasing transpiration

Limitations