7.7 - Xylem, transport of water

• What’s transpiration ?

  Process of evaporation of water at the spongy mesophyll of leaves, where water vapour is lost through the stomata
• Explain how water moves up the stem in the xylem, reference cohesion and tension

  • water evaporates from leaves, lowering water potential in the cells at the surface
  • water molecules from hydrogen bonds so they stick together, known as cohesion
  • water forms a continuous, unbroken column
  • as water evaporates, water is drawn up s a result of cohesion
  • the column is pulled up, transpiration pull
  • this puts the xylem under tension, there’s negative pressure in xylem
• 2 pieces of evidence that supports the cohesion-tension theory ?

  • Diameter - transpiration rate highest in the dat, tension is greatest, pulls diameter inwards
  • Broken xylem - pressure lost, cohesive forces break, water not pulled up, air drawn in rather than water leaking out