Movement in the Phloem (CIE 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:
  • On plants whose sap does ‘clot’ so that it is still possible to collect and study the 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