Transpiration Stream (Cambridge O Level Biology)

• During transpiration water molecules evaporate from the surface of mesophyll cells in the leaf before diffusing out of the leaf through the stomata
• When water is lost from leaves in this way, more water is drawn up the xylem to replace it; this movement of water in the xylem from the roots to the leaves of plants is the transpiration stream
• The transpiration stream is maintained by forces of attraction, known as cohesion, between water molecules as follows:
  • Water is lost from the leaves during transpiration
  • More water molecules are drawn into the leaf by osmosis
  • The moving water molecules 'pull' neighbouring water molecules up the xylem behind them due to forces of attraction between the molecules
  • A continuous column of water moves up the xylem
• The upward force generated by transpiration in the leaves is transpiration pull

Transpiration stream diagram

Water is drawn upwards in the xylem in a continuous column due to attractive forces between water molecules