Translocation - plants

  1. Sucrose loading mechanism to put sucrose into phloem at source

    • companion cells actively pump hydrogen ions out of cytoplasm into cell walls

    • hydrogen ions move down concentration gradient - carrying a sucrose molecule - back into cytoplasm via cotransporter protein

    • sucrose molecules move into sieve tubes via plasmodesmata

  2. High conc of solutes in phloem lowers water potential

    • water moves into phloem vessels via osmosis

    • can move in from neighbouring xylem vessels - hence why they are close together

  3. Increased hydrostatic pressure generates pressure gradient between source and sink

    • phloem contents (water + sucrose) moves down pressure gradient towards sink

  4. Sucrose is continually being unloaded from phloem at the sink - lowering water potential of sink cells

  5. Water follows water potential gradient - maintains hydrostatic pressure gradient between source and the sink