Movement in the Phloem (CIE A Level Biology)

• Translocation is 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 e.g. 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 (e.g. developing seeds, fruits or storage organs)

Sinks and Sources Diagram

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

• 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 ‘clots’, so that it is still possible to collect and study the sap (e.g. 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 (e.g. 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

Translocation Diagram

The translocation of assimilates from source to sink within phloem tissue