Transpiration (AQA A Level Biology): Revision Note

Water transport in plants

• A mass transport system is required in plants need a constant supply of water and minerals

  • Water is required for photosynthesis and to maintain cell structure

  • Minerals are needed for production of important biological molecules, e.g. proteins and chlorophyll

Xylem tissue

• Water and dissolved minerals are transported from the soil to the rest of a plant in the xylem

  • Xylem are tubes that form part of the mass transport system of plants

  • Xylem cells are specialised for water transport

    • Hollow tubes with no end walls allow the continuous flow of water

    • Lignin provides waterproofing to prevent loss of water by evaporation

    • Lignin strengthens the xylem to reduce breakages

• Xylem cells together form xylem tissue which, together with phloem tissue, makes up plant vascular tissue

Movement of water in the xylem

• The upward movement of water in the xylem is driven by the process of transpiration

  • Transpiration is the loss of water from the leaves of plants by evaporation

  • It is a form of mass transport

• Transpiration drives water transport as follows:

  1. water diffuses out of leaves into the surrounding air via the stomata

  2. the loss of water vapour lowers the water potential in the air spaces surrounding the mesophyll cells

  3. water within the mesophyll cell walls evaporates into the leaf air spaces, lowering the water potential of the mesophyll cells

  4. water is drawn from the xylem into the mesophyll cells by osmosis

  5. water moves up the xylem vessels in a continuous column to replace this lost water; this upward movement is the transpiration stream

     • Water molecules are pulled upwards due to forces of cohesion and adhesion

The cohesion-tension theory

• The upward pulling force acting on water in the xylem can be so great that the water is under tension, exerting an inward pull on the walls of the xylem vessels; this is known as cohesion tension

  • The mechanism of water movement described above is sometimes known as the cohesion-tension theory of water transport