Movement in the Phloem (Cambridge (CIE) AS Biology): Revision Note

Movement in the phloem

• 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)

• 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