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

• The source is where the organic compounds are synthesised – this is typically the photosynthetic tissues (leaves)

• The sink is where the compounds are delivered to for use or storage – this includes the roots, fruits and seeds

• Organic compounds are transported via a vascular tube system called the phloem as part of a viscous fluid called sap

Phloem Structure

Phloem sieve tubes are primarily composed of two main types of cells – sieve element cells and companion cells

• Plasmodesmata connect the sieve elements and companion cells to mediate symplastic exchange (continuous cytoplasm)
  

Sieve elements are the long and narrow cells that are connected together to form the phloem sieve tubes

• 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 cytoplasm and 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 elements and facilitate loading and unloading at sources and sinks

• They possess an infolding plasma membrane which increases SA:Vol ratio to allow for more material exchange
  

• Have many mitochondria to fuel the active transport of materials between the sieve tube and the source or sink
  

• Contain appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube
  

Sieve Tube Adaptations