transpiration pull

• Polar nature of water

  Helps the mass flow of water in a plant
• Hydrogen bonds (H-bonds)


  • Form between water molecules
  • Result in cohesion between water molecules
  • Result in adhesion between the cellulose in the cell walls and the water molecules
• Water movement in a plant

  1. Water moves from the roots to the leaves

  2. Due to a difference in water potential between the top and bottom of the plant

  3. Gradient present due to constant loss of water from the leaves by transpiration

  4. Constant uptake of water at the roots by osmosis
• Evaporation of water into the air spaces in the leaves


  Creates tension in the xylem tissue which is transmitted all the way down the plant
• Cohesive force of water


  • Results in a continuous column of water with high tensile strength
  • Adhesive force stops the water column from pulling away from the walls of the xylem vessels
• Cohesion-tension theory


  Mechanism for the movement of water in plants
• Xylem vessels


  • Have lignified walls to prevent them from collapsing due to the pressure differences created by the mass flow of water down its water potential gradient
• Transpiration stream


  Pathway of the water from the soil through the roots up the xylem tissue to the leaves
• Root pressure


  Plants aid the movement of water upwards by raising the water pressure in the roots
• Water entry into roots

  1. Water enters the roots down a water potential gradient from the surrounding soil

  2. Root cells actively transport solutes (e.g. mineral ions) from the cells of the root into the xylem vessels, lowering the water potential within the xylem

  3. Water is drawn into the xylem by osmosis from the surrounding cells, increasing the water pressure (root pressure)
• Apoplast pathway


  Water does not cross cell membranes, so does not move by osmosis
• Symplast pathway


  Water travels across the root
• Casparian strip


  • Waterproofs the cell walls of cells that surround the xylem
  • Forces any water flowing in the apoplast pathway into the symplast pathway
  • Passage through cell membranes provides greater control over the substances that are able to pass into the xylem
  • Contains a water-resistant molecule called suberin
• Although root pressure helps move water into the xylem vessels in the roots, it does not contribute greatly to the mass flow of water to the leaves in the transpiration stream
• Keywords for answering questions about transpiration

  • Water potential gradient (between leaves and roots)
  • Diffusion (of water vapour through the stomata)
  • Transpiration pull (evaporation of water from the mesophyll cells, of leaves, pulls other water molecules from the xylem tissue)
  • Cohesion (between water molecules)
  • Adhesion (between water molecules and cellulose within the cell walls)
  • Cohesion-tension theory (tension present in xylem vessels causes a continuous column of water and is due to cohesive and adhesive forces)
  • Osmosis (water moving via the symplast pathway across the roots and leaves)