Mass transport in plants

• Why are carbohydrates transported in the form sucrose?

  Because :

  • allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)
  • less reactive than glucose as its a non-reducing sugar and therefore no intermediate reactions occur as it is being transported)
• what does the mass flow hypothesis state

  translocation of sucrose and other sugars within the phloem occur via a continuous unidirectional flow of water and dissolved nutrient

  bulk flow of phloem sap is in:

  one direction

  Same rate

  same time
• Mass flow hypothesis

  • When the phloem sieve tube is punctured phloem sap oozes out
  • This suggests that it is under pressure
  • Phloem sap taken from near a source has a higher sucrose concentration than sap taken from near a sink
  • This suggests that different water potentials would result in osmosis into/out of the sieve tubes at those two locations
  • When a plant virus is applied to leaves that are well-lit the virus can be observed moving down the phloem towards the roots
  • This demonstrates the bulk flow of substances in one direction
  • When the virus is applied in the dark it is not transported
  • This suggests that for translocation to occur photosynthesis and the production of sucrose is required in the source tissue