Transpiration (AQA GCSE Biology: Combined Science)

What is transpiration?

• Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration
• Transpiration is defined as the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
• Xylem is adapted in many ways:
  • A substance called lignin is deposited in the cell walls which causes the xylem cells to die
  • These cells then become hollow (as they lose all their organelles and cytoplasm) and join end-to-end to form a continuous tube for water and mineral ions to travel through from the roots
  • Lignin strengthens the plant to help it withstand the pressure of the water movement

• Movement in xylem only takes place in one direction - from roots to leaves (unlike phloem where movement takes place in different directions)

Water uptake, transport and transpiration

• Air movement, humidity, temperature and light intensity all have an effect on the rate at which transpiration occurs
• The table below explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low

Transpiration rate factors table

Investigating the role of environmental factors in determining the rate of transpiration from a leafy shoot

• Cut a shoot underwater to prevent air entering the xylem and place in tube
• Set up the apparatus as shown in the diagram and make sure it is airtight, using vaseline to seal any gaps
• Dry the leaves of the shoot (wet leaves will affect the results)
• 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
• Set up the environmental factor you are investigating
• Allow the plant to adapt to the new environment for 5 minutes
• Record the starting location of the air bubble
• Leave for a set period of time
• Record the end location of the air bubble
• Change the light intensity or wind speed or level of humidity or temperature (only one - whichever factor is being investigated)
• Reset the bubble by opening the tap below the reservoir
• Repeat the experiment
• The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa

Investigating transpiration rates using a potometer

Environmental factors can be investigated in the following ways:

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

• Stomata can be opened or closed depending on the conditions the plant is in; this activity is controlled by guard cells
• The role of stomata and guard cells (found predominantly on the underside of the leaf) is to control gas exchange and water loss
• Guard cells have cell walls with unevenly distributed cellulose – the inner wall is thicker and the outer wall is thinner to aid opening and closing of the stomata
• When the availability of water is high, guard cells become turgid as a result of osmosis
  • When guard cells are turgid, the stomata they surround are open and air can circulate in from the environment but water is consequently lost via transpiration

• When less water is available, the guard cells lose water by osmosis and become flaccid
  • When guard cells are flaccid, they pull together, closing the stomata and reducing water loss via transpiration

• Stomata are predominantly distributed on the underside of the leaf where it is cooler and shaded (lower light intensity) – this leads to less transpiration and therefore less water loss

The guard cells control whether or not the stomata are open or closed, directly affecting how much transpiration can occur