transpiration and cohesion

• Transpiration


  Loss of water vapour from the stomata by evaporation
• Stomata


  • Guard cells that surge and bend, creating a pore where water vapour can evaporate out
  • Mainly found on the lower side of leaves
• Factors affecting rate of transpiration

  • Light intensity
  • Temperature
  • Humidity
  • Wind
• Light intensity increases


  Rate of transpiration increases
• Explanation for light intensity effect

  Higher light intensity causes more stomata to open, increasing surface area for water vapour to evaporate
• Temperature increases


  Rate of transpiration increases
• Explanation for temperature effect

  Higher temperature increases kinetic energy of water molecules, increasing rate of evaporation
• Humidity increases


  Rate of transpiration decreases
• Explanation for humidity effect

  Higher humidity reduces water potential gradient, decreasing ability for water to evaporate
• Wind increases


  Rate of transpiration increases
• Explanation for wind effect

  Wind carries away water vapour, maintaining water potential gradient and allowing more evaporation
• Cohesion-tension theory


  Explains how water can evaporate from leaves and be replaced by a continuous column moving up against gravity
• Cohesion


  • Water molecules stick together due to hydrogen bonding
  • Allows water to move up as a continuous column
• Adhesion


  • Water molecules stick to walls of xylem
  • Contributes to capillarity, drawing water up
• Capillarity


  Narrower xylem vessels increase adhesion, drawing water up further
• Root pressure


  • Positive pressure in roots pushes water column upwards
• Cohesion-tension theory explanation


  1. Transpiration creates lower pressure, pulling up water column

  2. Cohesion holds water column together

  3. Adhesion to xylem walls prevents water falling back

  4. Tension in xylem walls increases capillarity