The movement of water from roots to the leaves

• what direction does water always flow from roots to the leaves?

  down a water potential gradient
• why does water move from the soil, through the plant into the air in terms of water potential?

  as the air has a very low water potential and the soil water, a very dilute solution, has a very high water potential
• what are the 3 main mechanisms in which water moves from the soil, through the plant into the air?

  • cohesion-tension (transpiration)
  • capillarity
  • root pressure
• what is meant by the term cohesion?

  attraction of water molecules for each other, seen as hydrogen bonds, resulting from the dipole structure of the water molecule
• what happens during the mechanism of cohesion-tension?

  • in transpiration, water evaporates from leaf cells into the air spaces and diffuses out through stomata and into the atmosphere
  • this draws water across the cells of the leaf in the apoplast, symplast and vacuolar pathways, from the xylem
  • as water molecules leave xylem cells in the leaf, they pull up other water molecules behind them in the xylem
  • the water molecules all move as they show cohesion
  • this continuous pull produces tension in the water column
• what do the charges on the water molecules cause in cohesion-tension?

  the charges on the water molecules causes attraction to the hydrophilic lining of the vessels - this is adhesion, and contributes to water movement up the xylem
• what is meant by the term adhesion?

  attraction between water molecules and hydrophilic molecules in the cell walls of the xylem
• what does the cohesion-tension theory describe?

  it describes water movement up the xylem, by this combination of adhesion of water molecules and tension in the water column resulting from their cohesion
• what is cohesion-tension theory?

  the theory of the mechanism by which water moves up the xylem, as a result of their cohesion and adhesion of water molecules and the tension in the water column, all resulting from water’s dipole structure
• what is capillarity?

  the movement of water up narrow tubes by capillary action
• what happens during the mechanism of capillarity?

  • its the movement of water up narrow tubes, in this case the xylem, by capillary action
  • cohesion between water molecules generates surface tension and this, combined with their attraction to the walls of the xylem vessels (adhesion), draws the water up
  • capillarity only operates over short distances, up to a metre
  • it may have a role in transporting water in mosses, but only if it makes a small contribution of water movement in plants more than a few centimetres high
• what is root pressure?

  the upward force on water in roots, derived from osmotic movement of water into the root xylem
• what happens in the mechanism of root pressure?

  • root pressure operates over small distances in living plants and is a consequence of movement of water from the endodermal cells into the xylem pushing water already there further up
  • its caused by the osmotic movement of water down the water potential gradient across the root and into the base of the xylem
• what is this an image of?

  a diagram summarising water transport up a plant