Translocation

• what is translocation?

  • the movement of assimilates around a plant in the phloem (mainly sucrose but also amino acids)
• what are assimilates?

  • products of photosynthesis that are transported around the plant (e.g. sucrose, amino acids)
• what is the source?

  • regions of plants that produce assimilates by photosynthesis (e.g. leaves) or from storage materials (e.g. bulbs)
• what is the sink?

  • regions of the plant that require assimilates to supply their metabolic needs (e.g. roots, flowers, fruits)
• Translocation

  1. Sucrose produced in source

  2. H+ pump moves H+ out of companion cell by active transport

  3. H+ and sucrose enter companion cell together by facilitated diffusion using co-transporter protein

  4. Sucrose diffuses from companion cell into sieve tube element through plasmodesmata

  5. Sucrose lowers water potential so water moves in by osmosis

  6. High hydrostatic pressure in sieve tube at source

  7. Sucrose solution moves from area of high HP to area of low HP

  8. Sucrose removed from sieve element at sink by active transport by source pump

  9. Water leaves sieve tube by osmosis to lower water potential in xylem

  10. Water moves up xylem in transpiration stream
• when tree ringing occurs suggest why the trunk swells above the cut?

  • sugars can't pass cut (phloem was in the bark)
  • decrease water potential so water moves into cell
  • damage triggers - increased cell division to produce cells o store sugars
  • cut causes gall/infection
• what is the injecting part of an aphid called?

  stylet