9.2 Phloem Transport

• Translocation is the movement of organic compounds (e.g. sugars, amino acids) from sources to sinks

  • eds
• • The source is where the organic compounds are synthesised – this is the photosynthetic tissues (leaves)
• • The sink is where the compounds are delivered to for use or storage – this includes roots, fruits and seeds
• Organic compounds are transported from sources to sinks via a vascular tube system called the phloem
• • Sugars are principally transported as sucrose (disaccharide), because it is soluble but metabolically inert
  • The nutrient-rich, viscous fluid of the phloem is called plant sap
• Phloem sieve tubes are primarily composed of two main types of cells – sieve element cells and companion cells

  • The phloem also contains schlerenchymal and parenchymal cells which fill additional spaces and provide support
• Sieve elements are long and narrow cells that are connected together to form the sieve tube
• • Sieve elements are connected by sieve plates at their transverse ends, which are porous to enable flow between cells
• • Sieve elements have no nuclei and reduced numbers of organelles to maximise space for the translocation of materials
• • The sieve elements also have thick and rigid cell walls to withstand the hydrostatic pressures which facilitate flow
• Companion Cells

  Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at source and sink
• • Companion cells possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
• Companion cells have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
• • Companion cells contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube
• Sieve elements are unable to sustain independent metabolic activity without the support of a companion cell

  • This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)
  • Plasmodesmata exist between sieve elements and companion cells in relatively large numbers
  • These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites
• • In monocotyledons, the stele is large and vessels will form a radiating circle around the central pith
  • Xylem vessels will be located more internally and phloem vessels will be located more externally
• • In dicotyledons, the stele is very small and the xylem is located centrally with the phloem surrounding it
  • Xylem vessels may form a cross-like shape (‘X’ for xylem), while the phloem is situated in the surrounding gaps
• • In monocotyledons, the vascular bundles are found in a scattered arrangement throughout the stem
  • Phloem vessels will be positioned externally (towards outside of stem) – remember:  phloem = outside  
• • In dicotyledons, the vascular bundles are arranged in a circle around the centre of the stem (pith)
  • Phloem and xylem vessels will be separated by the cambium (xylem on inside ; phloem on outside)
• Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells

  • Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)
  • Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)
• Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure

  • Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)
  • The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient
  • Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement
  • This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source
• At the Source

  • The active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic
  • This causes water to be drawn from the xylem via osmosis (water moves towards higher solute concentrations)
  • Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase
  • This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)
  • Hence, the phloem transports solutes away from the source (and consequently towards the sink)
• At the Sink

  • The solutes within the phloem are unloaded by companion cells and transported into sinks
  • This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
  • Consequently, water is drawn out of the phloem and back into the xylem by osmosis
  • This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
  • Hence, phloem sap will always move from the source towards the sink
  • When organic molecules are transported into the sink, they are either metabolised or stored within the tonoplast of vacuoles
• Aphids can be used to collect sap at various sites along a plant's length and thus provide a measure of phloem transport rates
• • A plant is grown within a lab with the leaves sealed within a glass chamber containing radioactively-labelled carbon dioxide
  • The leaves will convert the CO2 into radioactively-labelled sugars (via photosynthesis), which are transported by the phloem
  • Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap
  • Once feeding has commenced, the aphid stylet is severed and sap continues to flow from the plant at the selected positions
  • The sap is then analysed for the presence of radioactively-labelled sugars
• • The rate of phloem transport (translocation rate) can be calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length
• Factors Affecting Translocation Rate

  The rate of phloem transport will principally be determined by the concentration of dissolved sugars in the phloem, affected by:

  • The rate of photosynthesis (which is affected by light intensity, CO2 concentration, temperature, etc.)
  • The rate of cellular respiration (this may be affected by any factor which physically stresses the plant)
  • The rate of transpiration (this will potentially determine how much water enters the phloem)
  • The diameter of the sieve tubes (will affect the hydrostatic pressure and may differ between plant species)