Water transport in plants

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  • Water transport in plants
    • Water moves into the root hair cells
      • by osmosis
        • because soil water has a high water potential and cytoplasm of root hair cells has a low water potential
      • evidence for the role of active transport in root pressure
        • some poisons, e.g. cyanide, prevent production of ATP
          • if cyanide is applied to root cells, root pressure disappears
        • temperature affects root pressure
          • suggests that chemical reactions are involved
    • Water moves across the root and into the xylem
      • Symplastic pathway
      • Apoplastic pathway
        • through apoplast - cell walls and intercellular spaces
          • by osmosis
            • because root hair cell has higher water potential than adjacent cell
              • water moves into adjacent cell, then this cell has higher water potential than next cell, so water moves to next cell etc.
              • water moves out of root hair cell, water potential of root hair cell drops, more water is taken up by osmosis - maintains steep concentration gradient
          • when water reaches the endodermal cells, movement os water is stopped by the waxy casparian *****
            • apoplastic pathway is inhibited and water is forced to move through the symplastic patyhway
    • Water moves up the xylem
      • Capillary action
        • water adheres to xylem
          • water moves upwards
      • Root Pressure
        • minerals are actively pumped into the xylem
          • this produces movement of water by osmosis (because it lowers the water potential of the xylem)
            • this increase in water is called root pressure
      • Transpiration stream (cohesion- tension theory)
        • water evaporates from leaves, out through stomata
          • lowers water potential in cell, water moves into cell  from adjacent cell by osmosis
            • this is repeated - water is drawn up the xylem in a continuous stream to replace the water that is lost - this is known as transpiration pull
              • water molecules move by adhesion and COHESION
                • this results in TENSION  in the xylem, helping to move the water up
        • Evidence for the cohesion tension theory
          • changes in the diameter of trees at different times of the day
            • high transpiration rate during the day,
              • high  tension in xylem
                • tree shrinks in diameter
          • if a xylem vessel is broken
            • air is pulled into the xylem
              • the plant can no longer move water up the stem as a continuous stream
                • because the cohesive forces between water molecules have been broken
        • Transpiration
          • the loss of water vapour from the plant as it evaporates from inside the leaf and diffuses down a concentration gradient out through stomata
          • Factors affecting transpiration
            • increase temperature
              • increase rate of photosynthesis
                • increase co2 demans
                  • stomata open
              • increase KE of water molesules
                • faster evaporation
            • increase light intensity
              • increase numbers of stomata open
                • increases rate of water vapour diffusing out of leaf
            • increase water availability
              • greater root pressure
            • increase numbers/ size of stomata
              • more water vapour can evaporate
            • increase wind
              • carries water vapour away
                • decreases water potential outside of plant
                  • increases water vapour potential gradient
                    • increase rate of transpiration
                      • more water vapour can evaporate
                      • greater root pressure
                      • increases rate of water vapour diffusing out of leaf
                      • faster evaporation
                      • stomata open
            • increase humidity
              • decrease water vapour potential gradient
                • decrease rate of transpiration

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