Xylem &Transpiration

• What is the definition of transpiration?

  Transpiration is the loss of water from the aerial parts of the plant (particularly from the stomata in the leaves) as a result of evaporation.
• Describe and explain the correlation between light intensity and transpiration.

  • Positive correlation
  • The higher the light intensity, the more stomata that open which provides a large surface area for evaporation.
• Describe and explain the correlation between temperature and transpiration.

  • Positive correlation
  • The higher the temp, the more kinetic energy which increases evaporation
• Describe and explain the correlation between humidity and transpiration.

  • Negative correlation
  • the more water vapour in the air, the more positive the water potential is outside of the leaf. This reduced the WP gradient and therefore reduces evaporation.
• Describe the explain the correlation between wind and transpiration.

  • Positive correlation
  • The windier it is, the more humid air is blown away. This maintains the WP gradient, increasing evaporation.
• Which properties of water allow cohesion-tension theory to occur?

  • Cohesion
  • Adhesion
• How does cohesion allow water molecules to travel up the xylem?

  • Water is a polar molecule.
  • Hydrogen bonds can form between the hydrogen and oxygen of different water molecules.
  • This creates cohesion between water molecules
  • So water travels up the xylem as a continuous water column.
• How does adhesion allow water molecules to travel up the xylem?

  • Adhesion of water is when water sticks to other molecules by forming hydrogen bonds.
  • Water adheres to the xylem walls.
  • This adhesion helps hold the water column up against gravity.
• Describe cohesion-tension theory.

  • As water evaporates out the stomata, this lowers pressure
  • water is pulled up xylem (due to negative pressure)
  • cohesive water molecules creates a column of water
  • water molecules adhere to walls of xylem pulling it upwards
  • this column creates tension, pulling xylem inwards
• Describe and explain how water moves up the xylem.

  • Water evaporates from the surface of mesophyll cells into air spaces in the leaf and then diffuses out through the stomata (transpiration)
  • The water potential of mesophyll cells decreases, so water is drawn in from neighbouring cells by osmosis, creating a pull on the water column in the xylem vessels. This pull creates tension in the xylem, drawing water up from the roots.
  • Cohesion allows water molecules to form a continuous column of water in the xylem vessels
  • At the same time, adhesion helps support the column and prevent it from collapsing under tension
  • These all create a continuous flow of water from the roots to the leaves