Final answer:
Sucrose produced in photosynthesis is actively transported into the phloem, reducing water potential and causing water to move into the phloem by osmosis from the xylem. This osmotic intake generates the pressure that drives the phloem sap from source to sink within the plant.
Step-by-step explanation:
Carbohydrates, primarily sucrose, are produced during photosynthesis and are transported throughout the plant via the phloem. When sucrose is actively transported into the sieve-tube elements of the phloem, it leads to a decrease in water potential. Consequently, water moves into the phloem by osmosis from the adjacent xylem. This osmotic movement of water generates a positive pressure that facilitates the bulk flow of the sucrose-water mixture from the source to the sink within the plant.
During the process known as phloem loading, not only is sucrose actively transported from source cells into the phloem, but the subsequent lower water potential in the phloem due to the high solute concentration encourages the osmotic flow of water into the phloem from the xylem. The increased turgor pressure within the phloem drives the bulk flow of phloem sap towards sinks, where sucrose is either used for growth, converted to starch for storage, or transformed into other polymers such as cellulose.
Phloem unloading at the sink occurs either through diffusion or active transport, and water that moved into the phloem by osmosis is then returned to the xylem, to eventually be transpired or recycled back into the phloem. This continuous cycle effectively distributes essential nutrients throughout the plant.