Final answer:
Active transport of sucrose into the phloem is conducted by the sucrose-H+ symporter, which uses ATP-derived energy to transport sucrose against its concentration gradient. This process is critical for the distribution of sucrose throughout the plant and for maintaining water balance between the phloem and xylem.
Step-by-step explanation:
The process of active transport of sucrose into the phloem involves the coupling of sucrose uptake with the electrochemical potential of a proton gradient. Sucrose molecules are transported against their concentration gradient, a mechanism facilitated by the sucrose-H+ symporter, a type of co-transport protein. This transport requires energy derived from ATP and is essential for the translocation of photosynthates, such as sucrose, from the source, where they are produced, to the sink, where they are utilized or stored.
During this process, sucrose is actively loaded from the mesophyll cells into companion cells and then into the sieve-tube elements of the phloem. This loading decreases the water potential inside the phloem, causing water to enter from the xylem, thus creating a positive pressure that aids in pushing the sucrose-water mixture downward towards the roots. At the sink, sucrose may be metabolized or stored, which ensures a gradient that favors the unloading process from the phloem.
The active transport of sucrose is not only crucial for the plant's internal distribution of essential nutrients but also plays a part in maintaining the plant's water balance through the generation of osmotic potential differences between xylem and phloem sap.