Water uptake + movement up the stem 2

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  • Created by: Steff06
  • Created on: 27-03-16 16:03
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  • Casparian strip and water movement in the stem
    • Role of the Casparian strip:
      • The Casparian strip BLOCKS the APOPLAST pathway between the cortex and the xylem.
        • This ensures water+nitrate ions have to pass into cell CYTOPLASM through cell membranes.
      • There are TRANSPORTER PROTEINS in the cell MEMBRANE. NITRATE ions are ACTIVELY TRANSPORTED from the cytoplasm of the cortex cells into the XYLEM.
        • The LOWER the water POTENTIAL in the XYLEM so water from CORTEX cells follows in the XYLEM by OSMOSIS.
          • Once water has entered the xylem, it cannot pass back into the cortex as the APOPLAST pathway of the endodermal cells is BLOCKED.
    • How does water move up the stem?
      • 3 processes that help move water up the stem: ROOT PRESSURE, TRANSPIRATION PULL and CAPILLARY ACTION.
      • ROOT PRESSURE: Action of endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis.
        • Root pressure can push water a few METRES up a stem, but cannot account for water getting to the top of tall trees.
      • TRANSPIRATION PULL: Loss of water by evaporation from leaves must be replaced by water coming up from the xylem. Water molecules are attracted to each other by COHESION.
        • COHESION forces are STRONG enough to hold molecules together in a long chain. This creates the TRANSPIRATION  STREAM. The pull from above can create TENSION in the column of water which is XYLEM vessels must be strengthened with LIGNIN.
          • Lignin prevents vessels from collapsing under tension (cohesion-tension theory). Relies on plant maintaining an UNBROKEN column of water all the way up the xylem.
            • If the water column is broken in 1 xylem vessel, the water column can still be maintained through another vessel via the PITS.
      • CAPILLARY ACTION: The same forces that hold water molecules together also attract the water molecules to the SIDES of the XYLEM vessel. This is called ADHESION,
        • Because the xylem vessels are very NARROW, these forces of attraction can pull the water up the sides of the vessel.
      • How water leaves the leaf:
        • Most of the water that leaves the leaf exits through the STOMATA which are TINY PORES in the EPIDERMIS.
        • Only a tiny amount leaves through the WAXY CUTICLE. Water evaporates from the cells lining the cavity below the GUARD cells.
          • This LOWERS the WATER POTENTIAL causing water to enter them by OSMOSIS from neighbourhing cells. Water enters these cells from cells DEEPER in the leaf until  water leaves the XYLEM and enters into the innermost leaf cells.

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