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  • Sucrose enters the phloem through an active process
  • ATP used by companion cells to actively transport H+ions out of cytoplasm
  • Sets up diffusion gradient for H+ions to diffuse back into companion cells
  • Diffusion occurs through co-transporter proteins: allow hions to bring back sucrose moleclues
  • Conc of sucrose increases in companion cell, diffuse through plasmodesmata into seive tube elements
  • At source, the water potentional in seive tube element reduces due to high conc of sucrose
  • Water molecules move into seive tube from surrounding cells by osmosis
  • Hydrostatic pressure increases
  • At sink, cells surrounding seive tube use sucrose for metabolic processes/starch
  • Reduces conc in cells
  • Sucrose moves out by diffusion/active transport to surrounding cells
  • This increases water potentional in seive tube, water moves out by osmosis to surrounding cells
  • Decreases hydrostatic pressure
  • Mass flow: The flow of water, from source to sink moving down a hydrostatic pressure gradient, which carries assimulates along phloem
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Evidence for translocation

  • Leafs: Produce sugars by photosynthesis, loaded in phloem once converted to starch, act as source, in summer. In early spring, need energy provided by stores in plant, act as sink
  • RootsStored carbohydrtaes released into phloem, in spring, source, During summer, store sugars as starch, sink
  • How do we know its Phloem?
    • if supplied with radioactively labelled CO2, labelled c appears in phloem
    • Ringing tree to remove bark (phloem in it), sugars collect above ring
    • Aphid feed on plant stem, mouth parts take food from phloem
  • How do we know its metabolic energy/ ATP?
    • Companion cells = lots of mitochondria
    • matabolic poison stops production of ATP= Translocation stop
  • How do we know its this mechanism?
    • pH of companion cells (hions)= higher than surrounding, conc of sucrose higher at source
  • Evidence against mechanism?
    • Not all solutes in phloem sap move at same rate
    • Sucrose moves to all parts of plant at same rate, rather than faster to areas of low conc
    • role of seive plates unclear
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