Phloem Lecture 2

  1. sugars transported in the phloem
    • sucrose (1 glucose + 1 fructose)
    • non-reducing sugars
    • raffinose (1 sucrose + 1 galactose)
    • stachynose
    • mannitol
    • sorbitol
  2. amino acids and amides transported in the phloem
    • glutamine
    • glutamate
    • aspartate
    • asparagine
  3. growth hormones transported in the phloem
    • auxin
    • gibberellin
  4. ions transported in the phloem
    K+, Mg2+, phosphate, Cl-

    not in the phloem = NO3-, Ca2+, B, SO42-, Fe
  5. specific mass transfer
    • unit cross-sectional area of phloem time
    • mass/time/cross-sectional are of phloem
  6. velocity
    • SMT = velocity x solute concentration
    • average = 100 cm/hr
  7. solute concentration in sieve tubes
    • measured using aphid mouth parts - insects that feed by inserting mouth parts into a single sieve element
    • high turgor pressure in the sieve element forces cell contents into insect's gut
    • sap is collected from aphid stylets cut off from body
  8. transport from chloroplast to sieve tube
    • chloroplast to cytosol - phosphate translocator
    • transport thru symplast - sucrose diffuses from cytosol from cell to cell via symplast vein
  9. symplastic pathway of phloem loading
    • move from mesophyll - intermediary cell - sieve tube through plasmodesmata
    • sucrose (M) ->sucrose = galactose = raffinose, stachyose (IC) -> ST
  10. apoplastic-symplastic pathway of phloem loading
    (apoplast) mesophyll -> (apoplast) companion cell -> sieve tube

    • sugars actively loaded from the apoplast
    • sugars loaded into companion cells move through plasmodesmata into sieve elements
  11. evidence for an apoplastic step
    • transport sugars are found in the apoplast
    • 14C-sucrose supplied to apoplast goes to companion cells and sieve tube
  12. transport from apoplast into the sieve tube
    • driven by proton gradient from H+ ATPase
    • sucrose- H+ symporter
  13. control of sucrose loading from the apoplast
    • affected by sucrose concentration - higher conc increases loading
    • regulated by turgor pressure - decrease increases loading
  14. evidence for active transport into the sieve tube
    • respiratory inhibitors - decrease ATP, inhibits loading
    • solute concentration (osmotic potentials) - differences can be measured, sugar must be moving against gradient
Card Set
Phloem Lecture 2
plant physiology and biochemistry exam 6