loading and unloading mechanism

• why are carbohydrates generally transported in the plants in the form of sucrose?

  it allows for efficient energy transfer and increased energy storage - sucrose is a disaccharide and thus contains more energy

  it's also less reactive than glucose as it's a non-reducing sugar and thus no intermediate reactions occur as it's being transported
• how may the sucrose molecules move to the sieve tube?

  by the symplastic pathway: through the cytoplasm and Plasmodesmata, which is a Passive process as the sucrose molecules move by diffusion

  by the Apoplastic pathway: through cell walls which is an Active process
• How do the sucrose molecules travel when taking the apoplastic pathway? p1

  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 thus 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 H+ ions move through a cotransporter protein
• p2 of sucrose molecules travelling the apoplastic pathway?

  while transporting the H+ 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

  this mechanism permits some plants to build up the sucrose in the phloem up to 3 times the concentration of that in the mesophyll
• What are adaptations of companion cells?

  companion cells have infoldings in their cell surface membrane to increase the available surface area for the active transport of solutes and also have many mitochondria to provide energy for the proton pump