Transpiration Stream (Cambridge (CIE) O Level Biology): Revision Note

Transpiration Stream

• 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