The Transpiration Stream (SL IB Biology)

• When water evaporates from the surfaces of cells inside a leaf during transpiration, more water is drawn from the nearest xylem vessels to replace the water lost by evaporation
  • Water molecules adhere to the cell walls of plant cells in the leaf, enabling water to move through the cell walls
    • Here the water moves through the cell walls of the xylem into other cells of the leaf
  • This movement of water that occurs due to adhesion to the walls of a narrow tube is capillary action
• The loss of water from the xylem vessels generates tension (negative pressure) within the xylem 
• The tension generated in the xylem when water moves into the cells in the leaves creates a pulling force throughout the xylem vessels that is transmitted, via cohesion between water molecules, all the way down the stem of the plant and to the ends of the xylem in the roots
  • This is known as transpiration pull and it allows water to be moved upwards through the plant, against the force of gravity
• This is sometimes known as the cohesion-tension theory of transpiration
• This continuous upwards flow of water in the xylem vessels of plants is known as the transpiration stream

Water transport in plants diagram

The movement of water through xylem vessels is due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules

• Transpiration is important to the plant in the following ways
  • It provides a means of cooling the plant via evaporative cooling
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants