Unit 1 Section 6.4 Translocation

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  • Translocation
    • What is Translocation?
      • Translocation is the movement of assimilates to where they're needed in a plant. its an energy - requiring process that happens in the phloem
      • Translocation moves assimilates from source s to sinks
        • the SOURCE is where assimilates are produced
        • the SINK is where assimilates are used up
        • Example; The Source for sucrose is the leaves, and the sinks are the other parts of the plant, especially the food storage organs and the meristems in the roots, stems and leaves
      • Enzymes maintain a concentration gradient from the source to the sink by changing the dissolved substances at the sink. this makes sure there's always a lower concentration at the sink than at the source
        • Example, In potatoes, sucrose is converted to starch in the sink areas, so there's always a lower concentration of sucrose at the sink than inside the phloem. this makes sure a constant supply of new sucrose reaches the sink from the phloem
    • The mass flow hypothesis
      • scientists still aren't certain exactly how the assimilates are transported from source to sink by translocation. the best supported therory is the mass flow hypothesis
        • 1. SOURCE active transport is used to actively load the assimilates into the sieve tubes of the phloem at the source
          • this lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis. this creates a high pressure inside the sieve tubes at the source end of the phloem
        • 2. SINK, at the sink end, assimilates are removed from the phloem to be used up.
          • This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis. this lowers the pressure inside the sieve tubes
        • 3. FLOW, The result is a pressure gradient from the source end to the sink end. This gradient pushes assimilates along the sieve tubes to where they're needed
    • Mass Flow Evidence
      • there is evidence both for and against mass flow
        • SUPPORTING EVIDENCE
          • 1), if you remove a ring of bark from a woody stem a bulge forms above the ring.
            • if you analyse the fluid from the bluge, you'll find it has a higher concentration of sugars than the fluid from below the ring
              • This is because the sugars can't move past the area where the bark has been removed - this is evidence that there's a downward flow of sugars
            • 2) you can investigate pressure in the phloem using aphids.
              • the sap flows out quicker nearer the leaves than further down the stem - this is evidence that there's a pressure gradient
              • 3) if you put a metabolic inhibitor into the phloem then translocation stops - this is evidence that active transport is involved
                • 4) there's an experimental model for mass flow
        • OBJECTIONS
          • sugar travels to many different sinks, not just to the one with the highest water potential, as the model would suggest
            • the Sieve plates would create a barrier to mass flow. a lot of pressure would be needed for assmilates to get through at a reasonable rate

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