Transpiration

• Mineral ions and organic compounds

  Transported in plants by being dissolved in water
• Xylem tissue


  Transports dissolved mineral ions
• Phloem tissue


  Transports dissolved organic compounds
• Plant roots


  • Responsible for uptake of water and mineral ions
  • Can have root hairs to increase surface area for absorption
• Uptake of water


  Passive process that occurs by osmosis (diffusion of water from higher to lower water potential)
• Uptake of minerals


  Can be passive or active, occurring by diffusion or active transport respectively
• Plants must take in constant supply of water and dissolved minerals to compensate for continuous loss via transpiration and enable photosynthesis and protein production
• Apoplastic pathway


  • Most water travels via this pathway (when transpiration rates are high), which is the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
  • Water moves by diffusion as it is not crossing a partially permeable membrane
  • Water can move from cell wall to cell wall directly or through the intercellular spaces
  • Movement occurs more rapidly than the symplastic pathway
• Casparian strip


  • Thick, waterproof, waxy band of suberin within the cell wall that blocks the apoplastic pathway when water reaches the endodermis
  • Thought to help the plant control which mineral ions reach the xylem and generate root pressure
• Symplastic pathway


  • Smaller amount of water travels via this pathway, which is the cytoplasm and plasmodesmata or vacuole of the cells
  • Water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata
  • Movement is slower than the apoplastic pathway
• Water (and dissolved substances) can travel

  From high water potential (soil) to low water potential (xylem) via the apoplastic or symplastic pathways
• Cohesion-tension theory


  Explains the movement of water through a plant's xylem, largely due to the evaporation of water vapor from the leaves and the cohesive and adhesive properties exhibited by water molecules
• Water potential gradient


  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
• Plants are constantly taking water in their roots and losing water via the stomata (in the leaves)
• Around 99% of the water absorbed is lost through evaporation from the plant's stem and leaves via transpiration
• Loss of water vapor from leaves (transpiration)

  Results in a lower water potential, creating a concentration gradient between the roots and leaves causing water to move upwards
• Movement of water through leaves

  1. Environmental conditions (e.g. low humidity, high temperatures) create a water potential gradient between the air inside the leaves (higher) and outside (lower)

  2. This results in water vapor diffusing out of the leaves through the stomata (transpiration)

  3. The water vapor lost lowers the water potential in the air spaces surrounding the mesophyll cells

  4. The water within the mesophyll cell walls evaporates into these air spaces resulting in a transpiration pull

  5. This transpiration pull results in water moving through the mesophyll cell wall (apoplastic pathway) or out of the mesophyll cytoplasm (symplastic pathway) into the cell wall

  6. The pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits (non-lignified areas), which then causes water to move up the xylem vessels (due to the cohesive and adhesive properties of the water) - this movement is called the transpiration stream
• Stomata


  • Pairs of guard cells that surround them
  • Open when guard cells are turgid, close when they lose water
  • When open, there is a greater rate of transpiration and gaseous exchange
  • When closed, transpiration and gaseous exchange decrease
  • Generally open during the day to allow CO2 in and O2 out