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b (i) Hydrogen ions are pumped out of the companion cells. This creates a gradient across the membranes of companion cells. There are Hydrogen ion channels present in the membranes of companion cells. These channels, being co-transporters, bring sucrose along with Hydrogen ions back to the cells. Due to the presence of numerous plasmodesmata b/w companion cells and sieve tube elements, the sucrose diffuses into C.
b (ii) Sucrose, when diffused into sieve tube elements, decreases the w.p of these 'elements'. Water enters these 'elements' by osmosis. On the other hand, at sinks, sucrose is either used up in respiration or stored as starch. These cells, therefore, have high water potential. Water moves out of these cells by osmosis. As a result of water entering at C and leaving at Sink, a hydrostatic pressure is created which causes a 'Mass Flow' of the sucrose solution along the phloem to the 'Sink'.
b (i) Hydrogen ions are pumped out of the companion cells. This creates a gradient across the membranes of companion cells. There are Hydrogen ion channels present in the membranes of companion cells. These channels, being co-transporters, bring sucrose along with Hydrogen ions back to the cells. Due to the presence of numerous plasmodesmata b/w companion cells and sieve tube elements, the sucrose diffuses into C.
b (ii) Sucrose, when diffused into sieve tube elements, decreases the w.p of these 'elements'. Water enters these 'elements' by osmosis. On the other hand, at sinks, sucrose is either used up in respiration or stored as starch. These cells, therefore, have high water potential. Water moves out of these cells by osmosis. As a result of water entering at C and leaving at Sink, a hydrostatic pressure is created which causes a 'Mass Flow' of the sucrose solution along the phloem to the 'Sink'.