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  • Translocation
    • Entering the Phloem
      • sucrose is actively loaded into the phloem
      • ATP is used by the companion cells to pump H+ out of their cytoplasm and into the surrounding tissue
        • setting up a diffusion gradient, so the hydrogen ions diffuse back into the companion cells
          • through co-transporter proteins (allows movement of 1 molecule linked to the movement of another in the same direction by active transport)
            • hydrogen brings sucrose molecules into the companion cells
              • as the concentration of sucrose builds up, they diffuse into the sieve tube elements, through plasmo-desmata
    • Along the Phloem
      • At the Sink
        • sucrose may be converted into starch for storage, or used in a metabolic process
          • this reduces the sucrose concentration in the cells that require sucrose
            • sucrose molecules diffuse or ATP is used for them to go from the sieve tube element to the cells
              • this increases the water potential of 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 carries sucrose and other assimilates along the phloem
            • MASS FLOW
            • up or down the plant, depending on where sugars are needed, sometimes in the same phloem tube at different times
      • At the Source
        • sucrose entering the sieve tube element reduces the water potential, so water moves into the sieve tube elements via osmosis from surrounding tissues
          • this increases the hydrostatic pressure in the sieve tube at the source
            • pressure created by fluids pushing against the sides of a container
    • the transport of assimilates (sugars and other chemicals made by plant cells), in the phloem tissue
      • transported in the phloem in the form of sucrose
      • part of the plant that releases sucrose into the phloem is called a source
        • .E.G. Leaves
          • sugars made during photo-synthesis are converted to sucrose and loaded into the phloem
      • part of the plant that removes sucrose from the phloem is called a sink
        • .E.G. Areas of growth, roots
    • Evidence
      • Needs ATP
        • companion cells have many mitochondria
        • translocation can be stopped when a metabolic poison that stops ATP formation is introduced
        • rate of flow of sugars in the phloem is o high that energy must be needed (up to 10000 quicker than diffusion)
      • Mechanism
        • pH of companion cells is higher than that in surrounding cells
        • concentration of sucrose is higher in source than sink
      • Phloem
        • if plant is given radioactive CO2, it soon appears in the phloem
        • ringing a tree to remove the phloem results in sugars collecting above the ring
        • an aphid  on a plant stem can be used to show that the mouthparts are taking food from the phloem
      • Negative Evidence
        • not all solutes in phloem sap move at same rate
        • role of sieve plates is unclear
        • sucrose is moved to all parts of the plant at the same rate, rather than going more quickly to parts with a low concentration


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