CIE A Level Biology (9700) 2019-2021

Revision Notes

7.2.2 Water & the Transpiration Pull

Water & the Transpiration Pull

The movement of water

  • Water has unique properties
    • it is polar
    • hydrogen bonds form between the water molecules
  • Water moves from the roots to the leaves because of a difference in the water potential gradient between the top and bottom of the plant. This gradient is created because of different events occurring within the plant and due to the properties of water
  • In the leaves, water evaporates from the mesophyll cells resulting in water (and any dissolved solutes) being pulled from the xylem vessels (transpiration pull) into the mesophyll cells
  • The water that is pulled into the mesophyll cells moves across them passively (either via the apoplastic – diffusion or symplastic – osmosis, pathways) lowering the hydrostatic pressure within the xylem vessels and creating a tension on these vessels
  • Xylem vessels have lignified walls to prevent them from collapsing due to the pressure differences being created from the mass flow (all the water molecules and any dissolved solutes move together) of water upwards
  • The mass flow is helped by the polar nature of water and the hydrogen bonds (H-bonds) that form between water molecules which results in cohesion between water molecules and adhesion between the cellulose in the cell walls and the water molecules
  • So due to the evaporation of water from the mesophyll cells in the leaves a tension is created in the xylem tissue which is transmitted all the way down the plant because of the cohesiveness of water molecules. The cohesive force results in a continuous column of water with high tensile strength (it is unlikely to break) and the adhesive force stops the water column from pulling away from the walls of the xylem vessels so water is pulled up the xylem tissue from the roots to replace what was lost in the leaves. This mechanism is called the cohesion-tension theory

The transpiration stream

  • The pathway of the water from the soil through the roots up the xylem tissue to the leaves is the transpiration stream
  • Plants aid the movement of water upwards by raising the water pressure in the roots (root pressure)
    • This is raised by actively secreting solutes (eg. mineral ions) into the xylem vessels in the root which lowers the water potential within the xylem
    • This results in water from the surrounding cells being drawn into the xylem (by osmosis) thus increasing the water pressure (root pressure)
  • Root pressure helps move water into the xylem vessels in the roots however the volume moved does not contribute greatly to the mass flow of water to the leaves in the transpiration stream

Exam Tip

When answering questions about transpiration it is important to include the following keywords:

  • Water potential gradient (between leaves and roots),
  • Diffusion (water vapour through the stomata)
  • Transpiration pull (evaporation of water from the mesophyll cells pulls other water molecules from the xylem tissue)
  • Cohesion (between water molecules)
  • Adhesion (between water molecules and cellulose within the cell walls)
  • Cohesion-tension theory (tension present in xylem vessels causes a continuous column of water and is due to cohesive and adhesive forces)
  • Osmosis (water via the apoplastic or symplastic pathways in the roots and leaves)

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.

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