Phloem: Mass Flow (CIE A Level Biology)

• The mass flow hypothesis was the model initially used to explain the movement of assimilates in the phloem tissue
• The simple model consisted of:
  • Two partially permeable membranes containing solutions with different concentrations of ions (one dilute the other concentrated)
  • These two membranes were placed into two chambers containing water and were connected via a passageway
  • The two membranes were joined via a tube
  • As the membranes were surrounded by water, the water moved by osmosis across the membrane containing the more concentrated solution
  • This forced the solution towards the membrane containing the more dilute solution (where water was being forced out of due to hydrostatic pressure)
• Scientists now support a modified version of this hypothesis – the pressure flow gradient

Mass Flow in Phloem Diagram

An illustration of Münch’s model for mass flow in phloem tissue

Pressure (hydrostatic) flow gradient

• Phloem sap (containing sucrose and other organic solutes) moves by mass flow up and down the plant
• The advantage of mass flow is that it moves the organic solutes faster than diffusion
• In xylem tissue the pressure difference that causes mass flow occurs because of a water potential gradient between the soil and leaf (this requires no energy input by the plant)
• However in phloem tissue energy is required to create pressure differences for the mass flow of the organic solutes
• The pressure difference is generated by actively loading sucrose into the sieve elements at the source (usually a photosynthesising leaf or storage organ) which lowers the water potential in the sap
• This results in water moving into the sieve elements as it travels down the water potential gradient by osmosis
• The presence of water within the sieve elements increases the hydrostatic or turgor pressure at the source and as solutes (e.g. sucrose) are removed / unloaded from the sieve elements causing water to follow by osmosis at the sink (creating a low hydrostatic pressure), a hydrostatic pressure gradient occurs
• The pressure difference between the source and the sink results in the mass flow of water (containing the dissolved organic solutes) from the high hydrostatic pressure area to the low hydrostatic pressure area
• The mass flow of organic solutes within the phloem tissue occurs above and below the sources (which is typically photosynthesising leaves). Therefore sap flows upwards and downwards within a plant

Mass Flow Diagram

The translocation of phloem sap (sucrose and other organic solutes) due to a hydrostatic pressure gradient from the source to the sink