Transportation of Food:

Phloem is responsible for transporting food substances throughout the plant. The glucose formed during photosynthesis in mesophyll cells is used in respiration and the excess of it is converted into sucrose. In most plants, the food is transported in the form of sucrose.

Pressure flow mechanism:

The currently accepted hypothesis states that the transport of food is through the pressure-flow mechanism. In the pressure-flow mechanism, the food is moved from sources to sinks.

Source:

The sources include any exporting organs typically a mature leaf or storage organ.

Sinks:

Sinks are the areas of active metabolism or storage, for example, roots, tubers, developing fruits and leaves, and the growing regions. A storage organ is capable of storing food and exporting stored materials. For example, the root of beet is a sink in the first growing season, but becomes a source in the next growing season, when sugars are utilized in the growth of new shoots.

Mechanism of Source:

At the source, the food (sugars) is moved by active transport into the sieve tubes of the smallest veins. Due to the presence of sugar in sieve tubes, their solute concentration increases, and water enters them from the xylem via osmosis. This results in higher pressure in these tubes, which drives the solution towards the sink.

Mechanism of Sink:

At the sink end, the food is unloaded by active transport. Water also exits from the sieve tubes. The exit of water decreases the pressure in sieve tubes, which causes a mass flow from the higher pressure at the source to the now lowered pressure at the sink. In other words, the mass flow is caused by drops in pressure at the sink as the food and water molecules are removed.