7.2.3 Water & the Transpiration Pull

The movement of water

• The mass flow of water in a plant is helped by the polar nature of water
  • Hydrogen bonds (H-bonds) form between water molecules which results in cohesion between water molecules and adhesion between the cellulose in the cell walls and the water molecules
• Water moves from the roots to the leaves because of a difference in water potential between the top and bottom of the plant.
  • This gradient is present due to the constant loss of water from the leaves by transpiration and the constant uptake of water at the roots by osmosis
• The evaporation of water into the air spaces in the leaves creates tension in the xylem tissue which is transmitted all the way down the plant because of the cohesive nature 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
  • This mechanism is called the cohesion-tension theory
• Xylem vessels have lignified walls to prevent them from collapsing due to the pressure differences created by the mass flow of water down its water potential gradient

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; this is known as root pressure
  • Water enters the roots down a water potential gradient from the surrounding soil
    • Minerals are transported into root cells by active transport, lowering the water potential of cells in the root
    • Water follows by osmosis
  • Root cells actively transport solutes (e.g. mineral ions) from the cells of the root into the xylem vessels; this lowers the water potential within the xylem
  • Water is drawn into the xylem by osmosis from the surrounding cells, thus increasing the water pressure (root pressure)
• Water travels across the root either via the apoplast pathway or the sympast pathway
  • Note that water in the apoplast pathway does not cross cell membranes, so does not move by osmosis
• A layer known as the Casparian strip waterproofs the cell walls of cells that surround the xylem; this forces any water flowing in the apoplast pathway into the symplast pathway, meaning that the water must travel through the cell membranes of the cells
  • Passage through cell membranes provides greater control over the substances that are able to pass into the xylem
  • The Casparian strip contains a water-resistant molecule called suberin  
• Root pressure helps move water into the xylem vessels in the roots, though it does not contribute greatly to the mass flow of water to the leaves in the transpiration stream

The transpiration stream involves the movement of water across the roots, up the xylem, and across the leaves of plants