transpiration

• Movement of water through a plant's xylem

  1. Evaporation of water vapour from the leaves

  2. Cohesive and adhesive properties exhibited by water molecules
• Water potential gradient


  The driving force behind the movement of water from the soil (high water potential) to the atmosphere (low water potential), via the plant's cells
• Plant water movement


  1. Constant water intake at roots

  2. Water loss via stomata in leaves

  3. Maintaining water potential gradient between roots and leaves
• Around 99 % of the water absorbed is lost through evaporation from the plant's stem and leaves in a process called transpiration
• Transpiration


  The loss of water vapour via the stomata by diffusion
• Transpiration is different to the transpiration stream which is the movement of water from the roots to the leaves
• Importance of transpiration to the plant

  • Provides a means of cooling the plant via evaporative cooling
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants
• Movement of water through leaves


  1. Water potential gradient between air inside leaves (higher) and air outside (lower)

  2. Water vapour diffusing out of leaves through stomata (transpiration)

  3. Water potential in air spaces surrounding mesophyll cells lowered

  4. Water evaporating from mesophyll cell walls into air spaces (transpiration pull)

  5. Water moving through mesophyll cell walls or out of mesophyll cytoplasm
• Apoplast pathway


  Movement of water through the cell walls of a plant
• Symplast pathway


  Movement of water from the cytoplasm of a cell
• Transpiration stream


  1. Water leaving xylem vessels through pits (non-lignified areas)

  2. Water moving up xylem vessels to replace lost water (due to cohesive and adhesive properties)
• When rates of transpiration are high the walls of the xylem are pulled inwards by the faster flow of water
• Stomata


  Pairs of guard cells that surround them
• Stomata control of transpiration

  1. Guard cells open stomata when turgid

  2. Guard cells close stomata when lose water

  3. Open stomata = greater transpiration and gaseous exchange

  4. Closed stomata = decreased transpiration and gaseous exchange
• Stomata allow gaseous exchange (CO2 in and O2 out) and are generally open during the day