Process of Transpiration

• The Process of Transpiration

  1. Plants are constantly taking water in their roots

  2. Plants are losing water via the stomata (in the leaves)

  3. Around 99% of the water absorbed by a plant is lost through evaporation from the plant's stem and its leaves in a process called transpiration
• Transpiration


  The loss of water vapour from a plant to its environment by evaporation and diffusion
• Transpiration


  Is a consequence of gaseous exchange at the stomata
• Advantage of transpiration


  • It provides a means of cooling the plant via evaporative cooling
  • The transpiration stream is helpful in the uptake of mineral ions
• Turgor pressure


  The pressure within the cells (due to the presence of water as it moves up the plant) that provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants
• Transpiration stream


  The movement of water from the roots to the leaves
• Evaporation of water vapour from the leaves

  And the cohesive and adhesive properties exhibited by water molecules causes the movement of water through a plant's xylem
• Gradient of water potential


  Is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant's cells
• Transpiration


  Only occurs when there is a concentration gradient, as there is usually a lower concentration of water molecules in the air outside the leaf
• Loss of water vapour from the leaves

  Results in a lower water potential creating a concentration gradient between the roots and leaves causing water to move upwards
• Factors affecting the rate of transpiration

  • Concentration gradient of water vapour between the inside of the leaf and the surrounding air
  • Air movement
  • Temperature
  • Light intensity
  • Humidity
• Larger concentration gradient


  Results in a faster rate of diffusion
• Air movement


  Can sweep water molecules away from the leaf surface, maintaining the concentration gradient and increasing the rate of transpiration
• Increase in temperature


  Increases the kinetic energy of molecules, therefore increasing the rate of transpiration as water molecules move out of the leaf down the concentration gradient at a faster rate
• High temperature


  Causes stomata to close to prevent excess water loss, dramatically reducing the rate of transpiration
• Stomata closure in the dark

  Greatly reduces the rate of transpiration
• Stomata opening when light is sufficient

  Increases the rate of transpiration
• High humidity


  Reduces the concentration gradient between inside the leaf and the outside air, causing the rate of transpiration to decrease
• High humidity


  Can reach an equilibrium where there is no concentration gradient and no net loss of water vapour from the leaves