Translocation (OCR A Level Biology)

• Although translocation could refer to the transport of substances in the xylem and phloem, as it means ‘moving from one place to another,’ it is more commonly connected with the transport of assimilates in the phloem tissue
• Thus translocation within phloem tissue can be defined as the transport of assimilates from source to sink and requires the input of metabolic energy (ATP)
• The liquid that is being transported (found within phloem sieve tubes) is called phloem sap
• This phloem sap consists not only of sugars (mainly sucrose) but also of water and other dissolved substances such as amino acids, hormones and minerals
• The source of the assimilates could be:
  • Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose, as sucrose has less of an osmotic effect than glucose)
  • Storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period)
  • Food stores in seeds (which are germinating)

• The sinks (where the assimilates are required) could be:
  • Meristems (apical or lateral) that are actively dividing
  • Roots that are growing and / or actively absorbing mineral ions
  • Any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs)

• The loading and unloading of the sucrose from the source to the phloem, and from the phloem to the sink is an active process
• It can be slowed down or even stopped at high temperatures or by respiratory inhibitors
• Translocation of assimilates is not fully understood yet by scientists. The understanding they do have has come from studies such as:
  • Collecting and studying the sap from plants with 'clotting' sap (eg. castor oil plants)
  • Using aphids to collect the sap – after the aphid inserts its stylet (tubular mouthpart) scientists remove the aphids head and collect the sap that continues to flow
  • Using radioactively labelled metabolites (eg. Carbon-14 labelled sugars) which can be traced during translocation
  • Advances in microscopes enabling the adaptations of companion cells to be seen
  • Observations about the importance of mitochondria to the process of translocation

Assimilates are moved through a plant by the process of translocation. They are moved from source to sink. Here are examples of sources and sinks

• 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

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 molecules. 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.