Arrange the following in correct sequence during transport of sugars through phloem from source (Leaf) to sink (Root).
A. Osmotic entry of water into xylem vessels from sieve tubes.
B. Loading of sugars into sieve tubes.
C. Unloading of sugars from sieve tubes into the storage cells of root.
D. Osmotic entry of water from into sieve tubes from xylem vessels.
As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a disaccharide). 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. This process of loading at the source produces a hypertonic condition in the phloem. Water in the adjacent xylem moves into the phloem by osmosis. 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.
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