Mass Flow Hypothesis

• Mass Flow Hypothesis


  Model initially used to explain the movement of assimilates in the phloem tissue
• Mass Flow


  1. Two partially permeable membranes containing solutions with different concentrations of ions

  2. Membranes placed into two chambers containing water and connected via a passageway

  3. Water moves by osmosis across the membrane containing the more concentrated solution

  4. Solution forced towards the membrane containing the more dilute solution

  5. Water forced out due to hydrostatic pressure
• Scientists now support a modified version of this hypothesis - the pressure flow gradient
• Pressure (hydrostatic) flow gradient

  Phloem sap (containing sucrose and other organic solutes) moves by mass flow up and down the plant
• Carbohydrates transported in plants as sucrose


  • Allows for efficient energy transfer and increased energy storage
  • Less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported
• Advantage of mass flow

  Moves the organic solutes faster than diffusion
• In xylem tissue, pressure difference that causes mass flow

  Occurs because of a water potential gradient between the soil and leaf, requires no energy input by the plant
• In phloem tissue, pressure difference that causes mass flow

  Requires energy to create, generated by actively loading sucrose into the sieve elements at the source which lowers the water potential in the sap
• Translocation of phloem sap

  1. Sucrose and other organic solutes move by mass flow due to a hydrostatic pressure gradient from the source to the sink

  2. Source is not necessarily the leaves, sink is not necessarily the roots

  3. Phloem sap moves up and down the plant, direction depends on the hydrostatic pressure gradient