MASS FLOW HYPOTHESIS

• MASS FLOW HYPOTHESIS?

  1. Sucrose actively transported from companion cells into STE
  2. Decreases water potential in phloem
  3. Water enters phloem by osmosis
  4. Increases hydrostatic pressure
  5. Hydrostatic pressure higher at source than sink
  6. Solutes move down pressure gradient to sink cell
  7. Solutes removed
  8. Water potential near sink increases
  9. Water moves out phloem by osmosis
  10. Maintains hydrostatic pressure gradient
• ACTIVE LOADING?

  1. Companion cells pump H+ out cytoplasm via proton pump using ATP
  2. Large concentration of H+ in cell wall of companion cells
  3. H+ diffuse down concentration gradient back into cytoplasm
  4. Move through co-transporter protein
  5. Also carries sucrose into companion cells against concentration gradient
  6. Sucrose enters sieve tubes via plasmodesmata
  7. High concentration of sucrose
  8. Decreases water potential in phloem
  9. Water enters phloem by osmosis
  10. Increases hydrostatic pressure (higher at source than sink)
  11. Mass flow of sucrose to sink where they are unloaded