The Mass Flow Hypothesis (AQA AS Biology): Revision Note

Translocation: mechanism

• The movement of organic compounds, e.g. sucrose, in the phloem is known as translocation

• The process by which phloem sap moves in one direction along phloem sieve tubes is known as the mass flow hypothesis

  • The direction may differ depending on the location of sources and sinks in the plant

• Translocation occurs as follows:

  1. active transport is used to load organic compounds into the phloem at the source

     • Companion cells use ATP to actively pump hydrogen ions out of the cytoplasm into their cell walls

     • Hydrogen ions move down their concentration gradient back to the cytoplasm via a cotransporter protein, carrying sucrose molecules

     • Sucrose molecules then move into the sieve tubes via plasmodesmata

  2. the high concentrations of solutes in the phloem lower the water potential and cause water to move into the phloem vessels by osmosis

     • Water can move in from the neighbouring xylem vessels

  3. this results in increased hydrostatic pressure and generates a hydrostatic pressure gradient between the source and the sink; the contents of the phloem move towards the sink down a pressure gradient

  4. at the same time sucrose is being unloaded from the phloem at the sink, lowering the water potential of the cells of the sink

  5. water follows by osmosis, maintaining the hydrostatic pressure gradient between the source and the sink