Movement in the Phloem (AQA A Level Biology): Revision Note

Movement in the phloem

• In addition to water and minerals from the soil, plants also need to transport organic substances

  • E.g. carbohydrates produced in the leaves by photosynthesis will be needed for respiration in other parts of the plant

• Organic substances, also known as assimilates, are transported in the phloem; examples include

  • sucrose

  • amino acids

  • plant hormones

• Phloem tissue contains examples of specialised cells

  • Phloem sieve tube cells

    • Reduced cell contents to reduce resistance to flow of assimilates

    • Sieve plates allow passage of assimilates between cells

  • Companion cells

    • Contain many mitochondria to produce ATP for the active loading of sucrose into the phloem tubes

• Movement of assimilates through the phloem is translocation

• Translocation moves assimilates either upwards or downwards from cells in the source to cells in the sink

  • A source of assimilates is the place in which it has been produced or stored, e.g.:

    • cells in photosynthesising leaves are a source of sugars

    • cells in storage organs during the early spring may be a source of carbohydrates for new growth

  • A sink is the part of a plant where assimilates are required, e.g.:

    • cells in parts of a plant that are actively growing

    • cells in plant storage organs

• Translocation is an active process, dependent on energy from ATP