transpiration and cohesion-tension theory

• what is transpiration?

  the movement/loss of water vapour as it evaporates from the stomata
• what are the four conditions that effect the rate of transpiration?


  1. humidity
  2. temperature
  3. wind
  4. light intensity
• how does light intensity effect transpiration?

  • positive correlation
  • more light means more stomata open
  • higher surface area for the evaporation of water
• how does temperature effect transpiration?

  • positive correlation
  • more heat
  • more kenetic energy
  • so faster moving molecules and more evaporation
• how does humidity effect transpiration?

  • negative correlation
  • more water vapour in the air
  • higher water potential in the air than the leaf
  • reduces the water potential gradient
• how does wind effect transpiration?

  • positive correlation
  • blows away the water vapour and humid air
  • maintaining water potential gradient
• what are the three parts of the cohesion-tension theory?

  1. cohesion
  2. capillarity (adhesion)
  3. root pressure
• what is cohesion?

  • water is a dipolar molecule
  • water molecules stick to each other due to hydrogen bonds
  • water molecules travel together up the xylem as a continuous column
• what is capillarity?

  • adhesion of water molecules
  • water molecules stick to other molecules such as the walls of the xylem
  • the narrower the xylem the bigger the impact if capillarity
• what is root pressure?

  • as water moves into the roots via osmosis it increases volume and therefore pressure
  • this pushes water up
• what is a potometer?

  measure water uptake
• when using a potometer, how do you prepare the plant?

  by cutting a part of the plant underwater
• why do you cut the plant underwater?

  because xylems have negative pressure and will draw up air and break the column of water
• how should a potometer be filled?

  with no air bubbles
• how do u prevent air bubbles in a potometer?

  by using rubber seals and petroleum jelly for no water leakages and air bubbles
• why would one air bubble be in the potometer?

  to measure the distance it travels on the capillary tube
• how do you measure the rate of transpiration?

  volume / distance