Final answer:
The phloem becomes hypertonic to the xylem because it actively transports sucrose, which reduces its water potential and leads to water influx by osmosis, increasing pressure and driving the flow from source to sink.
Step-by-step explanation:
The phloem becomes hypertonic to the xylem due to the active transport of sucrose into it. This active transport involves the movement of sucrose into the phloem's sieve-tube elements (STEs) against its concentration gradient, which requires energy in the form of ATP. This process is facilitated by a carrier protein known as the sucrose-H+ symporter, which couples the transport of sucrose with the electrochemical potential of the proton gradient.Once the sucrose is loaded into the phloem, it increases the solute concentration, thereby decreasing the water potential (referred to as Ys). As the water potential drops, water moves from the xylem, which has a higher water potential, into the phloem by osmosis. This influx of water raises the internal pressure within the phloem, and this positive pressure is what drives the bulk flow of the sucrose-water mixture from sources, such as photosynthesizing leaves, to sinks where sucrose is either used for growth, converted to starch, or other polymers.At the sink cells, sucrose may be unloaded by either diffusion or active transport, and the water diffuses out of the phloem, either being recycled back via the xylem or lost through transpiration. These processes are crucial for the transport of carbohydrates, mainly sucrose, from the sites of production to other parts of the plant where they are needed.