Phloem Lecture 3

  1. pressure flow hypothesis for transport through the sieve tube
    • 100 cm/hr is too fast for diffusion
    • driving force for bulk flow is the hydrostatic pressure gradient
    • gradient maintained by phloem loading at the source and phloem unloading at the sink
    • sieve plates create a resistance
  2. evidence for pressure flow hypothesis
    • demonstration of adequate pressure gradients
    • demonstration of open sieve plate pores - rapid freezing and fixation
    • demonstration of bulk flow particle movement - viruses
    • mathematical modeling supports bulk flow through the sieve tube
  3. phloem unloading
    • sieve tube unloading
    • short distance transport
    • storage and metabolism in receiver cell
  4. symplastic unloading
    • young leaves
    • sugar beet
    • tobacco
    • primary root tips
  5. apoplastic unloading
    • in developing leaves of monocots - few plasmodesmata
    • cell wall invertase
    • sucrose -> gluctose + fructose
    • sugar-H+ symporter
  6. active membrane transport into receiver cells
    • sucrose-proton antiporter - sugar is transported to the vacuoles
    • vacuolar H+ ATPase pumps protons into vacuole
Card Set
Phloem Lecture 3
plant physiology and biochemistry exam 6