Answer:
The Pressure Flow or Mass Flow Hypothesis The accepted
mechanism used for the translocation of sugars from source to sink is called
the pressure flow hypothesis. As glucose is prepared at the source (by
photosynthesis) it is converted to sucrose (a dissacharide). The sugar is then
moved in the form of sucrose into the companion cells and then into the living
phloem sieve tube cells by active transport.
As osmotic pressure builds up, the phloem sap will move to
areas of lower pressure. At the sink, osmotic pressure must be reduced. Again
active transport is necessary to move the sucrose out of the phloem sap and
into the cells which will use the sugar converting it into energy, starch or
cellulose. As sugars are removed, the osmotic pressure decreases and water
moves out of the phloem.
Hydrostatic pressure in the phloem sieve tube increases,
pressure flow begins and the sap moves through the phloem. Meanwhile, at the
sink, incoming sugars are actively transported out of the phloem and removed as
complex carbohydrates. The loss of solute produces a high water potential in
the phloem and water passes out, returning eventually to xylem.
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