3.6.5 The Sucrose Loading Mechanism

• Assimilates such as sucrose are transported from source to sink through the phloem sieve tubes
• Carbohydrates are generally transported in plants in the form of sucrose because:
  • 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 (eg. sucrose)

• The pathway that sucrose molecules use to travel to the sieve tubes is not fully understood yet. The molecules may move by the:
  • symplastic pathway (through the cytoplasm and plasmodesmata) which is a passive process as the sucrose molecules move by diffusion
  • apoplastic pathway (through the cell walls) which is an active process

• If the sucrose molecules are taking the apoplastic pathway then modified companion cells (called transfer cells) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls. This is an active process and therefore requires ATP as an energy source
• 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
• The hydrogen ions move through a cotransporter protein. While transporting the hydrogen ions this protein also carries sucrose molecules into the companion cell against the concentration gradient for sucrose
• 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 and 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
• A high concentration of sucrose decreases the water potential in the phloem and water enters by osmosis
• The entry of water results in a high pressure (pressure gradient) which enables the mass flow of sugars towards sink tissues
• At the sink tissues, sugars are unloaded

Unloading of assimilates (eg. sucrose)

• The unloading of the assimilates (eg. sucrose) occurs at the sinks
• Scientists believe that the unloading of sucrose is similar to the loading of sucrose, with the sucrose being actively transported out of the companion cells and then moving out of the phloem tissue via apoplastic or symplastic pathways
• To maintain a concentration gradient in the sink tissue, sucrose is converted into other storage molecules such as starch. This is a metabolic reaction so requires enzymes (eg. invertase which hydrolyses sucrose into glucose and fructose)

The apoplast and symplast pathways used when sucrose is loaded into the phloem tissue. The enlarged portion of the companion cell shows the proton and co-transporter proteins used to actively load the sucrose.

• The intracellular space and walls of the sieve cells are adapted for the mass transport of sugars
• There are very few organelles, a small volume of cytoplasm and a large vacuole present in the intercellular space
  • The phloem sap can flow easily by moving through the open corridor that has low resistance

• Sieve cells have thick cell walls
  • These walls help them to withstand the pressure exerted by the mass flow of sugars