Transport of Water in the Xylem

• What is transpiration?

  The loss of water vapour from stomata by evaporation.
• What four factors affect the rate of transpiration?

  1. Light intensity
  2. Temperature
  3. Humidity
  4. Wind
• How does an increase in light intensity affect transpiration?

  It increases the rate of transpiration as more stomata are open, so the surface area for transpiration is increased.
• How does an increase in temperature affect transpiration?

  It increases the rate of transpiration. The water molecules have more kinetic energy so move faster and evaporate sooner.
• How does an increase in humidity affect the rate of transpiration?

  It decreases the rate. This is because the air outside the leaf has a higher water potential so the water potential gradient is reduced.
• How does an increase in wind speed affect the rate of transpiration?

  It increases the rate because the wind blows away humid air, maintaining the water potential gradient.
• Why is the movement of water up the xylem difficult?

  It goes against gravity.
• What are the three factors of the cohesion-tension theory?

  • Cohesion
  • Adhesion
  • Root pressure
• Explain cohesion.

  • Water is a dipolar molecule.
  • Hydrogen bonds form between oxygen and hydrogen atoms.
  • This creates cohesion.
  • Water travels up the xylem as a continuous column.
• Explain adhesion.

  • Water adheres to the walls of the xylem.
  • The narrower the xylem, the greater the impact of adhesion.
• Explain root pressure.

  • As water moves into the roots by osmosis, the volume increases so the pressure increases.
  • This increase in pressure forces water above it upwards (positive pressure).
• How does water move up the xylem?

  1. Water evaporates out of stomata. Loss in water volume= lower pressure.
  2. More water is pulled up to replace lost water. It moves due to negative pressure.
  3. Water molecules are cohesive due to hydrogen bonds between them. This creates a column of water in the xylem.
  4. Water molecules adhere to the walls of the xylem. This helps to pull water upwards.
  5. As the column is pulled upwards, it creates tension, pulling the xylem in to be narrower.