9.2.2 Sucrose Loading

• Organic compounds such as sucrose are transported from source to sink through the phloem sieve tubes
  • Sucrose is a disaccharide formed from a molecule of glucose and a molecule of fructose
  • Carbohydrates are transported in the form of sucrose because this compound cannot directly enter the respiration reactions and so it is less likely to be respired while in the process of being transported
• The pathways that sucrose molecules take when being loaded into phloem sieve tubes at the source include
  • The apoplast, or apoplastic, pathway
    • Through the cell wall spaces
    • This is an active process as ion pumps are involved
  • The symplast, or symplastic, pathway
    • Through the cytoplasm and plasmodesmata 
      • Plasmodesmata (singular plasmodesma) are channels through the cell walls of plant cells that connect neighbouring cells
    • This is a passive process as the sucrose molecules move by diffusion

The apoplast pathway

• Sucrose molecules that move along the apoplast pathway enter the sieve tube as a result of the following process
  • Companion cells pump hydrogen ions out of their cytoplasm into their cell walls
    • This pumping is carried out by proteins known as proton pumps
    • This is an active process and therefore requires ATP
  • The high concentration of hydrogen ions in the cell wall space of the companion cell results in the hydrogen ions moving down their concentration gradient back into the cytoplasm 
  • The hydrogen ions move via a co-transporter protein, meaning that whilst transporting the hydrogen ions, this protein also carries sucrose molecules into the companion cell against the sucrose concentration gradient
  • The sucrose molecules then move into the sieve tubes from the companion cells by diffusion through plasmodesmata
• Note that this process takes place inside a specialised type of companion cell known as a transfer cell; these cells have a folded cell surface membrane to increase the available surface area for the active transport of solutes and many mitochondria to provide the energy for the proton pumps
• This mechanism permits plants to build up the sucrose concentration in the phloem to many times more than that in the surrounding cells

The symplast pathway

• In some plants sucrose molecules mainly travel from the cytoplasm of one cell to the cytoplasm of neighbouring cells by diffusion through plasmodesmata

Sugar can enter the sieve tube cells via either the apoplast or symplast pathway.  Entry via the apoplast pathway involves proton pump and co-transporter proteins.