3.4.3.1 Transpiration

• Transpiration - loss of water vapour from the stomata via evaporation
• Factors Affecting Transpiration

  • Light Intesity
  • Humidity
  • Wind
  • Temperature
• Affect of Light Intensity on Transpiration

  • Positive Correlation
  • Higher LI = Faster rate of transpiration
  • Because more light causes more stomata to open
  • Causing a larger surface area for evaporation
• Affect of Temperature on Transpiration

  • Positive Correlation
  • Higher temperature = more kinetic energy
  • Water vapour molecules move faster
  • Rate of evaporation increases
• Affect of Humidity on Transpiration

  • Negative Correlation
  • More water vapour in air = increase water potential of air
  • Therefore decreases water potential in leaf and evapouration
• Affect of Wind on Transpiration

  • Positive Correlation
  • More wind = blow away humid air
  • Maintains water potential gradient
• In a plant the water potenial is highest/more positive at the roots
• Water moves up the plant from the roots against gravity
• Cohesion-Tension Theory

  1. Cohesion
  2. Capillarity - adhesion
  3. Root Pressure
• Cohesion

  • Water is a polar molecule
  • This allows hydrogen bonds to form between hydrogen and oxygen of different water molecules
  • This creates cohesion between water molecules and they stick together
  • Therefore water travels up the xylem as a continuous water column
• Capillarity

  • Adhesion (water molecules sticking to other molecules)
  • Water adheres to the xylem wall
  • The narrower the xylem, the bigger the impact of capillarity
• Root Pressure

  • Water moves into roots = increases volume of water in roots
  • Therefore pressure inside roots increases
  • Root pressure = force moving water upwards (positive)
• How does water move up the xylem tube?

  1. Water vapour evapourates out of the stomata, causing a decrease in water volume & pressure (transpiration)
  2. More water is pulled up the xylem to replace it (moves due to negative pressure)
  3. Cohesion (due to hydrogen bonding) causes water molecules to stick together and creates a water column up the xylem tube
  4. Water molecules adhere to the walls of the xylem allowing the water column to pull upwards
  5. As the water column pulls up the xylem it creates tension, making the column narrower (capillarity)
• How can the xylem form a column?

  • Xylem cells are hollow because they're dead
  • Xylem cells have no end walls
• Potometer = measures rate of uptake of water by plants; used to measure the rate of transpiration because they are proportional
• Potometers can be used to investiagte the effect of a named environmental variable on the rate of transpiration
• Rate of transpiration = (volume of water OR distance bubble moves) / time taken for bubble to move
• Sample of plant must be cut from a plant underwater to prevent any air from entering the xylem and breaking the water column
• Using a rubber seal and petroleum jelly ensures the potometer is air tight