Explain pressure flow hypothesis of translocation of sugars in plants.
The pressure flow hypothesis states that food is prepared in the plant leaves in the form of glucose. For translocation to the other parts the glucose is converted to sucrose a disaccharide. The sucrose is moved into the companion cells and then into the living sieve tube cells by active transport.
This process is called loading and creates a hypertonic condition in the phloem. Water from the adjacent xylem moves into the phloem by osmosis. Increasing osmotic pressure results in the movement of phloem sap to areas of lower pressure. To reduce the osmotic pressure at the sink the sucrose is moved out of the phloem by active transport. The sucrose is converted to energy, starch or cellulose. The removal of the sugars lowers the osmotic pressure resulting in the movement of water out of the phloem.