active loading

• Active loading is used at the source to move substances into the companion cells from surrounding tissues, and from the companion cells into the sieve tubes, against a concentration gradient. The concentration of sucrose is usually higher in the companion cells than the surrounding tissue cells, and higher in the sieve tube cells than the companion cells
• Sucrose is moved to where it needs to go using active transport and co-transport proteins. Co-transport proteins are a type of carrier protein that bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient. In active loading, H+ ions are used to move sucrose against its concentration gradient.
• ATP transport in companion cell


  1. ATP used to actively transport hydrogen ions out of the cell and into surrounding tissue cells

  2. Sets up a concentration gradient - more H+ ions in the surrounding tissue than in the companion cell
• Sucrose transport in companion cell

  1. H+ ion binds to co-transport protein in companion cell membrane and re-enters the cell (down the concentration gradient)

  2. Sucrose molecule binds to the co-transport protein at the same time

  3. Movement of the H+ ion is used to move the sucrose molecule into the cell, against its concentration gradient
• Sucrose transport out of companion cells


  Sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process
• The breakdown of ATP supplies the initial energy needed for the active transport of the H+ ions.