the mass flow hypothesis

• 1.source

  active transport is used to actively load the solutes (e.g. sucrose from photosynthesis) into the sieve tubes of the phloem at the source (e.g. leaves)

  • this lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells
  • creates a high pressure inside sieve tubes at the source end of the phloem
• 2. sink

  • at the sink end, solutes are removed from the phloem to be used up
  • usually happens by diffusion because solutes are at a higher concentration in the phloem then in the surrounding tissue at the sink
  • removing solutes increases water potential inside the sieve tubes so water also leaves the tubes by osmosis lowering the pressure inside the sieve tubes
• 3. flow

  • result is a pressure gradient from the source end to the sink end
  • this pushes solutes along the sieve tubes towards the sink
  • when reach sink solutes will be used or stored
  • higher concentration of sucrose at the source = higher rate of translocation