• 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.