mass flow hypothesis

• translocation is the process by which products of photosynthesis are transported from a source to a sink
• translocation is carried out by the phloem tissue which is made from sieve-tube elements and their companion cells.
• sources are almost always leaves as this is where the majority of photosynthesis takes place, sinks can be any part of the plant that has a high energy demand
• Mass flow hypothesis is the theory that explains the movement of water and solutes from a high to low concentration across a selectively permeable membrane
• mass flow hypothesis relies on creating pressure and creating a concentration gradient
• (1) source - releases sucrose into phloem

  high sucrose concentration, sucrose moved into sieve tubes by active loading. Decreasing water potential so that water moves in by osmosis from xylem increase in volume increases turgor pressure
• (2) Sink - removes sucrose from phloem

  low sucrose concentration as sucrose moves out of sieve tubes down a concentration gradient as cells use it or convert to glucose.

  increases water potential in sieve tubes so water moves back to xylem by osmosis, reduction in volume decreases turgor pressure
• (3) mass flow of solutes

  turgor pressure gradient created between source and sink causes solutes to be pushed in that direction.

  concentration gradient of sucrose between source and sink created as sucrose constantly being used for respiration or converted for storage
• Active loading is the process in which sucrose is actively transported into sieve tube elements
• Active loading steps:

  • H+ ions are actively pumped out of the companion cell into cells of the source tissue
  • this creates a hydrogen concentration gradient across the companion cell membrane
  • H+ ions can diffuse back down the gradient through special transport proteins which require a sucrose molecule to be co-transported
  • this increases the concentration of sucrose in the companion cells which can then diffuse into the sieve tube
• evidence to support mass flow hypothesis:

  • sucrose concentration is higher in the leaves than the roots
  • preventing ATP production stops translocation but not water movement ( lack of ATP for active transport in companion cells )
  • if a ring of bark is removed from a woody stem then a bulge forms above the ring - its a build up of fluid that cannot continue. theres more sugar in the fluid above the bulge then below, evidence for downward flow of sugars