1.

Explain Mass flow hypothesis

Answer»
  • The mechanism used for the translocation of sugar from source to sink end is called pressure flow or mass flow. 
  • Glucose converted into sucrose then moved into the companion cells and then to sieve tube cells of phloem. 
  • This process of loading at the source form the hypertonic condition in the phloem. 
  • Water from the adjacent xylem moves into the phloem by osmosis. Osmotic pressure builds up in the phloem sap moves to area of lower pressure. 
  • At the sink osmotic pressure must be reduced. The movement of sugar in the phloem begins at the source, where sugars are loaded into a sieve tube. 
  • Loading of phloem set up a weaker potential gradient that facilitates the mass movement in the phloem. As hydrostatic pressure increased in the sieve tube, pressure flow begins and the sap moves through the phloem. Mean while at the sink incoming sugars are actively transported out of the phloem. The loss of solute reduces a high water potential in the phloem and water passes out returning eventually to xylem. 


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