Transpiration (AQA GCSE Biology): Revision Note

Transpiration: Principles

• Transpiration can be defined as:

the loss of water vapour from the leaves of plants via the stomata

• In order for transpiration to occur, water:

  • exits the xylem in the leaf

  • evaporates from the internal surfaces of the leaf into the air spaces

  • diffuses out of the stomata in the form of water vapour

• The flow of water from the roots, up the xylem, to the leaves and out into the air is called the transpiration stream. It is driven by evaporation from the leaves and does not need energy from respiration

Transpiration: Factors

• Air movement, humidity, temperature and light intensity all affect the rate at which transpiration occurs

Transpiration: Investigating

Aim:

• Estimate the rate of transpiration by measuring the uptake of water in a leafy shoot

  • This can be determined using a piece of equipment called a potometer

Method:

1. Cut a shoot underwater

   • This prevents air bubbles from entering the xylem

2. Set up the apparatus as shown in the diagram above and make sure it is airtight, using petroleum jelly to seal any gaps

   • This should also be done underwater to prevent entry of air bubbles

3. Dry the leaves of the shoot using a paper towel

   • Wet leaves will affect the results because water can block the stomata

4. 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

5. Set up the environmental factor that is being investigated

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

7. Record the starting location of the air bubble

8. Leave for a set period of time

9. Record the end location of the air bubble

   • The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa

   • Rate of water uptake = volume of water taken up ÷ time taken (e.g. cm³/min). This is used as an estimate of the rate of transpiration

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

11. Change the experimental factor being investigated: light intensity, wind speed, level of humidity or temperature

12. Repeat the experiment

• Environmental factors can be investigated as follows:

  • Airflow: set up a fan or hairdryer

  • 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: cold room or warm room

Transpiration: Stomata

• Stomata (singular: stoma) are pores located mainly on the underside of leaves

• They are surrounded by specialised guard cells which control whether stomata are open or closed

• Stomata open during the day to maximise gas exchange for photosynthesis

• A side-effect of this is that water vapour is lost by transpiration

• In dry conditions plants may keep their stomata closed to preserve water

• At night there is no light, so no photosynthesis; the stomata close, which reduces water loss. As light intensity increases, the stomata open wider to take in more carbon dioxide for photosynthesis