Movement of water up through roots

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  • Created by: Laura
  • Created on: 04-05-14 14:21
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  • Movement of water through roots
    • Uptake of water by root hairs
      • Root hairs are the exchange surfaces in plants that are responsible for the absorption of water and mineral ions
      • Each root hair is long, thin extensions of a root epidermal cell. These root hairs remain functional for a few weeks before dying back to be replaced
      • They are efficient surfaces for the exchange of water and mineral ions
        • They provide a larger surface area as they are very long extensions and occur in thousands on each of the branches of roots
        • They have a thin surface layer which materials can cross easily
      • After water has been absorbed into the root hair cell, water continues its journey across the root in two ways
        • the apoplastic pathway
        • The symplastic pathway
    • The apoplastic pathway
      • This is the non-living pathway
      • As water is drawn into epidermal cells it pulls water along behind it due to cohesive properties
      • This creates a tension that draws water along the cell walls of the cells of the root cortex.
      • The mesh like structure of cellulose cell walls of these cells has many water filled spaces and so there is little or no resistance to this pull of water along the cell walls
    • The symplastic pathway
      • Movement of water through living cells
      • Takes place across the cytoplasm of the cells of the cortex as a result of osmosis
      • The water passes through the cell walls along tiny openings called plasmodesmata. Each one is filled with a thin strand of cytoplasm
      • Water entering by osmosis increases the water potential of the root hair cell
        • Water potential in root hair cell now higher than that of the first cell of the cortex
          • Moves to first cell inn the cortex by osmosis down the water potential verdict
            • First cell now has a higher water potential than the neighbouring cell
    • Passage of water into the xylem
      • When water reaches the endodermis by the apoplastic pathway the waterproof band that makes up the casparian strip which stops it going further than the cell wall
        • Water forced into the living portoplast of the cell where it joins water that has arrived via the symplastic pathway
      • Active transport of salts is is the most likely mechanism by which water now gets into the xylem
        • This process requires energy so can only occur within living tissue. It takes place within carrier proteins in the cell surface membrane
          • If water is to enter the xylem of the endodermal cells by the casparian strip
  • The symplastic pathway

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