Water uptake and movement up the stem

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  • Water uptake and movement up the stem
    • Water uptake from the soil
      • The outermost layer of cells (the epidermis) contains root hair cells that increase the surface area of the root.
        • These cells absorb minerals from the soil by active transport using ATP for energy.
      • The minerals reduce the water potential of the cell cytoplasm. Makes the water potential lower in the soil.
      • Water is taken up across the plasma membrane by osmosis as the molecules move down the water potential graiden
    • Movement across the root
      • The movement of water across the root is driven by an active process that occurs at the endodermis.
        • The endodermis is a layer of cells surrounding the xylem.
        • Also known as the starch sheath as it contains granules of starch (a sign that energy is being used)
        • The endodermis consists of special cells that have a waterproof strip in some of their walls, called the Casparian strip
          • The Casparian strip blocks the apoplast pathway, forcing water into the symplast pathway
      • The endodermis cells move minerals by active transport from the cortex into the xylem
        • This decreases the water potential in the xylem. As a result, water moves from the cortex through the endodermal cells to the xylem by osmosis
          • Reduces the water potential in the cells just outside the endodermis.
            • This combined with water entering the root hair cells, creates a water potential gradient across the whole cortex.
              • Therefore water is moved along the symplast pathway from the root hair cells, across the cortex and into the xylem
      • At the same time, water can move through the apoplast pathway across the cortex.
        • This water moves into the cells to join the symplast pathway just before passing through the endodermis.
    • What is the role of the Casparian strip?
      • The Casparian strip blocks the apoplast pathway between the cortex and the xylem
      • This ensures that water and nitrate ions have to pass into the cell cytoplasm through all cell membranes
      • There are transporter proteins in the cell membranes.
      • Nitrate ions are actively transported from the cytoplasm or the cortex cells into the xylem
      • This lowers the water potential in the xylem so water from cortex cells follows into the xylem by osmosis
      • Once the 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?
      • Root Pressure
        • The action of the endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis
        • This forces water into the xylem and pushes the water up the xylem.
        • Root pressure can push water a few metres up a stem, but can't account for water getting to the top of tall trees
      • Transpiration pull
        • The loss of water by evaporation from the leaves must be replaced by water coming up from the xylem.
        • Water molecules are attracted to each other by forces of cohesion.
          • These cohesion forces are strong enough  to hold the molecules together in a long chain or column
            • As molecules are lost at the top of the column, the whole column is pulled up as one chain. Creates the transpiration stream
        • The pull from above can create tension in the column of water.
          • This is why the xylem vessels must be strengthened by lignin.
            • The lignin prevents the vessel from collapsing under tension
          • Called the cohesion-tension theory
            • Relies on the plant maintaining an unbroken column of water all the way up the xylem.
      • Capillary action
        • The same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessel. Called adhesion
        • Because the xylem vessels are very narrow, these forces of attraction can pull water up the sides of the vessel
    • How water leaves the leaf
      • Most of the water that leaves the leaf  exits through the stomata
      • Only a tiny amounts leaves through the waxy cuticle.
      • Water evaporates from the cells lining the cavity immediately below the guard cells
        • This lowers the water potential in these cells, causing water to enter them by osmosis from neighbouring cells.
          • Water then enters these neighbouring cells from deeper in the leaf, and so on until eventually water leaves the xylem and enters the innermost leaf  cells


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