The Mass Flow Hypothesis (OCR A Level Biology): Revision Note

Mass flow in the phloem

• Phloem sap, containing sucrose and other organic solutes, moves by mass flow up and down the plant

  • Carbohydrates are generally transported in plants in the form of sucrose because:

    • sucrose is a disaccharide and therefore contains more energy than a monosaccharide

    • sucrose is less reactive than glucose as it is a non-reducing sugar

• Mass flow occurs due to the presence of a hydrostatic pressure gradient

  • The pressure gradient is generated by actively loading sucrose into the sieve elements at the source; this lowers the water potential in the sieve tube

  • Water moves into the sieve elements by osmosis; this increases the hydrostatic pressure at the source

  • At the same time, solutes are unloaded from the sieve elements at the sink, causing water to follow by osmosis; this lowers the hydrostatic pressure at the sink

  • The difference in hydrostatic pressure between the source and the sink creates a hydrostatic pressure gradient

• The pressure difference between the source and the sink results in the mass flow of phloem sap from the high hydrostatic pressure area to the low hydrostatic pressure area

• Mass flow means that solutes move faster than they could move by diffusion alone