Mass Flow Hypothesis

• what does the mass flow hypothesis do?


  Best explains phloem transport - best supported theory of how the solutes are transported from source to sink by translocation
• STEP 1 OF THE MASS FLOW HYPOTHESIS?


  1. Active transport used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source
  2. this lowers the water potential inside the sieve tubes, so water enters tubes by osmosis from xylem and companion cells
  3. = creates a high pressure inside the sieve tubes at the source end of the phloem
• STEP 2 OF THE MASS FLOW HYPOTHESIS?

  1. At sink end, solutes removed from phloem to be used up
  2. = this increases the water potential inside the sieve tubes = so water also leaves tubes by osmosis
  3. = lowers pressure inside sieve tubes
• STEP 3OF THE MASS FLOW HYPOTHESIS?

  1. the result = a pressure gradient from the source end to the sink end
  2. this gradient pushes solutes along the sieve tubes towards the sink
  3. when they reach the sink the solutes will be used or stored
• what have experiments shown?

  Some sucrose is transported also through the cell walls of the phloem