The movement of sugars - translocation

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  • The movement of sugars - translocation
    • Translocation
      • The movement of assimilates (sugars and other chemicals made up plant cells)
      • Sugars are transported in the phloem in the form of sucrose
      • A part of the plant that releases sucrose into the phloem is called a source
      • A part of the plant that removes sucrose from the phloem is called a sink
    • How does sucrose enter the phloem
      • Sucrose is loaded into the phloem by an active process.
        • ATP is used by the companion cells to actively transport hydrogen ions out of their cytoplasm and into the surrounding tissues.
          • This sets up a diffusion gradient and the hydrogen ions diffuse back into the companion cells.
            • This diffusion occurs through cotransporter proteins, these enable the hydrogen ions to bring sucrose molecules into the companion cells.
              • As the concentration of sucrose molecules builds up inside the companion cells, they diffuse into the sieve tube elements through the plasmodesmata
    • Movement of sucrose along the phloem
      • At the source
        • Sucrose entering the sieve tube element reduces the water potential inside the sieve tube
        • As a result, water molecules move into the sieve tube element by osmosis from surrounding tissues
        • This increases the hydrostatic pressure in the sieve tube at the source
      • At the sink
        • Sucrose is used in the cells surrounding the phloem.  The sucrose may be converted to starch for storage, or may be used in a metabolic process
        • This reduces the sucrose concentration in these cells. Sucrose molecules move by diffusion or active transport from the sieve tube element into the surrounding cells.
        • This increases the water potential in the sieve tube element, so water molecules move into the surrounding cells by osmosis.
        • This reduces the hydrostatic pressure in the phloem at the sink
      • Along the phloem
        • Water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving the phloem at the sink, produces a flow of water along the phloem.
        • This flow carries sucrose and other assimilates along the phloem. Called mass flow. It can occur in either direction (up or down the plant) depending on where the sugars are needed
    • Evidence for this mechanism of translocation
      • How we know the phloem is used:
        • if a plant is supplied with radioactively labelled carbon dioxide, the labelled carbon soon appears int the phloem
        • ringing a tree to remove the phloem results in sugars collecting above the ring
        • an aphid feeding on a plant stem can be used to show the mouthparts are taking food from the phloem
      • How we know it needs ATP:
        • companion cells have many mitochondria
        • translocation can be stopped by using a metabolic poison that inhibits the formation of ATP
        • the rate of flow of sugars in the phloem is so high that energy must be needed to drive the flow
      • How we know it uses this mechanism:
        • the pH of the companion cells is higher that that of surrounding cells
        • the concentration of sucrose is higher in the source than in the sink
      • Is there evidence against this mechanism:
        • not all the solutes in the phloem sap move at the same rate
        • sucrose is moved to all parts of the plant at the same rate, rather than going more quickly to areas with a low concentration
        • the role of sieve plates is unclear


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