Final answer:
Sugars produced in the leaves are actively transported into phloem sieve-tube elements using ATP and a sucrose-H+ symporter. The sugar's presence reduces water potential and creates pressure flow in the phloem, moving the sugars toward the plant's sinks like roots or shoots.
Step-by-step explanation:
In the translocation of sugars, sugars made in photosynthetic cells are pumped into phloem sieve-tube elements by active transport at the source. This process involves the loading of sugars, primarily sucrose, from the mesophyll cells into companion cells and then into the phloem STEs against their concentration gradient. This active transport process requires energy in the form of ATP and utilizes the electrochemical potential of the proton gradient, coupling the uptake of sucrose with a carrier protein known as the sucrose-H+ symporter.
The high concentration of sucrose in the phloem decreases the water potential, causing water to move by osmosis from the adjacent xylem into the phloem tubes. This results in an increase in pressure that facilitates the bulk flow of the sugar-water mixture from the source to the sinks, such as roots, young shoots, and developing seeds. At the sink, sucrose is either metabolized for growth or converted to starch for storage.