water uptake and movement up stem (incomplete)

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  • Created by: Lois
  • Created on: 08-01-13 12:09
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  • Water uptake and movement in stem
    • water uptake from soil
      • plant roots - surrounded by soil particles. outermost layer of cells (apidermis) contains root hair cells that increase surface area
        • these absorb minerlas from soil by active transport using ATP for energy
          • minerals reduce water potential of cell cytoplasm - makes water potential in cell lower than soil
            • water taken up across plasma membrane doing water potential grandient by osmosis
    • cohesion: attraction of water molecules
    • adhesion: attraction of water molecules in walls of xylem
    • movement across root
      • driven by active process that occurs in endodermis - layer of cells surrounding xylem, also know as startch sheath; sign energy being used
        • endodermis consists of special cells that have waterprrof strip in some of there walls: casparian strip
          • casparian strip blocks apoplast pathway forcing water through symolast pathway
            • endodermis cells move mineral by active transport from cortex into xylem: decreasing xylem water potential
              • so water moves by osmosis, reducing wp in cells outside endodermis. this combined with water entering root hair cells creates wp gradient across cortext
                • water moved along symplast pathway from root hair cells across cortex into xylem. at same time water can move across apoplast pathway across cortex
                  • -water moves into cells to join symplast pathway just perfore passing through endodermis
    • role of casparian strip
      • blocks apoplast pathway, ensures water and dissolves nitrate ions pass into cytoplas, through cell membranes
        • transporter proteins in cell membranes, nitrate can be actively transported - from cytoplasm of cortex cells to xylem
          • lowers wp in xylem so water from cortex cells follows into xylem by osmosis.
            • once water entered xylem it cannot pass back into cortex as apoplast pathway of endodermal cells blocked
    • how water up stem


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