The Roots & Water Transport (HL) (HL IB Biology)

• Loss of water at the leaves by transpiration creates a negative pressure inside xylem vessels which pulls water up the stem of plants, but plants can also push water up from the roots by generating root pressure
• Root pressure is generated as follows:
  • Minerals are actively transported into the cells of the roots from the soil, lowering the water potential of the root cells
    • Note that a low water potential is the result of a high solute concentration
  • This creates a water potential gradient between the soil and the cells of the roots and water moves into the root cells by osmosis
  • Minerals are then actively transported from cell to cell across the root, eventually reaching the xylem
  • As minerals enter the cells and the xylem the water potential is reduced and water follows by osmosis
  • Water enters the xylem and generates a positive pressure potential; this pushes the column of water upwards; this is root pressure
• Root pressure is important in driving the movement of water up xylem vessels when the rate of transpiration is low
• • E.g. in conditions of high humidity or before the leaves of deciduous plants develop in spring

Root pressure diagram

Water moves into the xylem by osmosis due to the water potential gradient created when minerals are actively transported into roots from the soil

Water that enters the xylem in this way generates root pressure