Final answer:
Sucrose is the primary solute in the phloem sap of plants, facilitating the transport of nutrients by creating a water potential gradient that causes bulk flow from source to sink within the plant's phloem system.
Step-by-step explanation:
Sucrose is the most prevalent solute in phloem sap, playing a crucial role in the transport of photosynthates within plants. Phloem sap, an aqueous solution, can contain up to 30 percent sugar, including minerals, amino acids, and plant growth regulators. At the source, sucrose is actively transported into companion cells and then into sieve-tube elements, significantly reducing the water potential and causing osmotic movement of water from the xylem into the phloem. The resulting pressure facilitates the bulk flow of phloem sap from source to sink areas, where sucrose concentrations are lower due to its use in metabolic processes or conversion to storage or structural molecules like starch or cellulose. Water then diffuses from the phloem, likely to be later transpired or recycled back into the xylem.