water transport in plants

• Xylem vessels transport water and mineral ions up the plant and provides support
• Xylem adaptations:

  • a hollow lumen allows for more space for water etc to pass through
  • lignin that forms the walls is strong and prevents the tube from collapsing
  • gaps in lignin allow for water to leave the vessels
  • lignin is waterproof to prevent water from adhering to the surface
• Xylem structure:

  • elongated tubes formed from dead cells
  • no end walls between cells
  • thick walls made from lignin
  • gaps called pits in the cell wall
• What is cohesion-tension theory?

  water molecules form hydrogen bonds with each other causing them to 'stick' together
• Cohesion-tension steps:

  1. water evaporates (transpiration)
  2. cells now have negative water potential so water diffuses in through osmosis
  3. the increase in water tension pulls water into the leaf ( transpiration pull)
  4. as the water molecules are pulled into the leaf others follow, from the roots to the mesophyll tissue
  5. water enters the stem through the roots
• evidence to support cohesion tension:

  • if a trunk or stem is damaged and a xylem cell is broken water doesn't leak out but once air enters the trunk the tree can no longer draw up water because the continuous column of water has been broken
  • trunks of trees reduce in diameter during the day, this is due to transpiration. This is because adhesion to water molecules results in tension which pulls the xylem walls in. At night when transpiration is lower there's less tension so diameter increases.