Water Uptake and movement up the Stem

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  • Water Uptake & Movement up the Stem
    • From the Soil
      • the outermost layer of cells in a plant root contain root hair cells that increase the surface area of the root
        • root hair cells absorb minerals from the soil by active transport using ATP for energy
          • they reduce the water potential of the cell cytoplasm, making it lower than the water potential of the soil, so water is taken up across the plasma membrane by osmosis
    • Movement across the root
      • the endodermis cells move minerals by active transport from the cortex into the Xylem
        • this decrease the water potential of the xylem, causing water to move into it
          • this reduces the water potential in the cells just outside the endodermis, creating a water potential gradient across the whole cortex (due to the water entering  the root hair cells)
            • water is moved along the symplast pathway from the root hair cells, across the cortex and into the Xylem
      • driven by an active process occurring  in the endodermis (a layer of cells around the Xylem)
        • known as the starch sheath as it contains granules of starch - a sign that energy is being used
        • consists of special cells that have a waterproof strip in some of their walls (Casparian Strip)
          • strip blocks the apoplast pathway, forcing water into the symplast pathway
    • Casparian Strip
      • blocks the apoplast pathway between the cortex and the Xylem, ensuring that the water and nitrate ions have to pass into the cytoplasm through cell membranes
        • there are transporter proteins in the cell membranes, allowing nitrite ions to be actively transported from the cytoplasm of the cortex into the Xylem
          • lowering the water potential in the Xylem so water moves from the cortex in to the Xylem via osmosis
            • the water cannot pass back into the cortex because the apoplast pathway of the endodermal cells is blocked
    • Up the Stem
      • Transpiration Pull
        • Water molecules are attracted to eachother by forces of cohesion - forces strong enough to hold molecules together in a long column
          • as molecules are lost at the top of the column, the whole thing is pulled up 1 chain, creating a Transpiration Stream
            • the pull from above can create tension in the column of water, lignin must be used to strengthen the Xylem Vessels and stop them from collapsing under the tension
        • Cohesion-Tension theory relies on the plant maintaining and unbroken column of water all the way up the Xylem
          • if the column is broken in one vessel, it can be maintained through another vessel via pits
      • Root Pressure
        • the action of the endodermis moving minerals into the Xylem by active transport drives water into the Xylem by osmosis
          • forcing the water into the Xylem pushes it up the Xylem, up to a few metres upwards (not far enough to get to the top of a tree)
      • Capillary Action
        • the forces that hold water molecules together also attract the water molecules to the sides of the Xylem vessel (ADHESION)
          • due to the narrow vessels, these forces can pull the water up the sides of the vessel
    • Leaving the Leaf
      • most water leaves through the Stomata, tiny pores in the epidermis
        • water evaporates from the cells lining the cavity immediately below the gaurd cells
          • this lowers the water potential in these cells, causing water to enter them by osmosis from   neighboring cells
            • water enters the neighbouring cells from deeper in the leaf and so on until water leaves the Xylem and enters the innermost leaf cells
      • a tiny amount leaves through the waxy cuticle

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