sucrose

• Sucrose Loading Mechanism


  The process by which sucrose is transported from source to sink through the phloem sieve tubes
• Carbohydrates are generally transported in plants in the form of sucrose
• Reasons why sucrose is used for transport


  • It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)
  • It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported
• Loading of assimilates


  1. Symplastic pathway (through the cytoplasm and plasmodesmata)

  2. Apoplastic pathway (through the cell walls)
• Apoplastic pathway loading


  1. Modified companion cells (called transfer cells) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls

  2. The large concentration of hydrogen ions in the cell wall of the companion cell results in the hydrogen ions moving down the concentration gradient back to the cytoplasm of the companion cell

  3. The hydrogen ions move through a cotransporter protein which also carries sucrose molecules into the companion cell against the concentration gradient for sucrose

  4. The sucrose molecules then move into the sieve tubes via the plasmodesmata from the companion cells
• Companion cells


  • Have infoldings in their cell surface membrane to increase the available surface area for the active transport of solutes
  • Have many mitochondria to provide the energy for the proton pump
• This mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that in the mesophyll
• Unloading of assimilates


  1. Sucrose is actively transported out of the companion cells

  2. Sucrose then moves out of the phloem tissue via apoplastic or symplastic pathways

  3. Sucrose is converted into other molecules to maintain a concentration gradient in the sink tissue

  4. This is a metabolic reaction so requires enzymes (e.g. invertase which hydrolyses sucrose into glucose and fructose)