Translocation in Plants (HL) (HL IB Biology)

• Translocation is the biological term used to describe the transport of organic solutes in the phloem tissue
  
  • The liquid that is being transported within the phloem can be referred to as phloem sap
  • This phloem sap consists of sugars in the form of sucrose, along with water and other dissolved substances such as amino acids
    • These dissolved substances are sometimes referred to as assimilates
• Translocation within phloem tissue transports these organic compounds from regions known as ‘sources’ to regions known as ‘sinks’

Sources and sinks

• Sources are the regions of plants in which organic solutes originate; they can include
  
  • Mature green leaves and green stems
    • Photosynthesis in these regions produces glucose which is converted into sucrose before being transported
  • 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
• Sinks are the regions of plants where organic compounds are required for growth; they can include
  • Meristems that are actively dividing
  • Roots that are growing or actively taking up mineral ions
  • Young leaves in bud
  • Any part of the plant where organic compounds are being stored, e.g. developing seeds, fruits, or storage organs
• Note that sources can become sinks and vice versa, depending on the time of year and the processes taking place inside the plant

Sources and sinks diagram

Organic compounds are moved through a plant, by the process of translocation, from source(s) to sink(s).

Phloem adaptations

• The function of phloem tissue in a plant is to transport organic compounds, particularly sucrose, from sources to sinks
• The cells that make up the phloem tissue are highly specialised, meaning that their structure aids their function
• Phloem is a complex tissue made up of different cell types; it is mainly made up of sieve tube elements and companion cells
  • Sieve tube cells, or elements, line up end-to-end to form a continuous tube through which phloem sap flows
    • The cells are separated by perforated sieve plates which allow the passage of assimilates
    • Sieve tube cells have reduced cytoplasm and few organelles to allow the free flow of phloem sap
  • Companion cells are closely associated with the sieve tube and aid with the loading and unloading of dissolved substances, or assimilates
    • Companion cells contain many mitochondria which generate ATP for the active loading of sucrose into the sieve tube
• Sieve tubes and companion cells are linked by bridges of cytoplasm known as plasmodesmata

Phloem adaptations diagram

Phloem tissue contains sieve tube cells and companion cells

Sieve tube structure and function table

Companion cell structure and function table

Translocation

• The loading and unloading of sucrose and other organic compounds from the source to the phloem, and from the phloem to the sink is an active process, meaning that it requires energy in the form of ATP
• A summary of the process is
  1. Active transport is used to load organic compounds into the phloem at the source
  2. The high concentrations of solutes in the phloem at the source lower the water potential and cause water to move into the phloem vessels by osmosis
     • Water can move in from the neighbouring xylem vessels
  3. This results in a raised hydrostatic pressure and generates a hydrostatic pressure gradient between the source and the sink; this causes the contents of the phloem to flow towards the sink down a pressure gradient
     • Hydrostatic pressure refers to the pressure exerted by a fluid on the walls of its container; in this case the walls of the phloem
  4. At the same time sucrose is being unloaded from the phloem at the sink, lowering the water potential of the cells of the sink
  5. Water follows by osmosis, maintaining the hydrostatic pressure gradient between the source and the sink