Illustrating Pressure-flow hypothesis
The pressure flow hypothesis of food movement states that dissolved sugars flow from sources and are released at sinks where they are used. This means that dissolved sugars enter the phloem at sources (such as the mesophyll layer of leaves) and move through the phloem until they reach a sink (such as a growing root) where they are needed.
The accepted mechanism needed for the translocation of sugars from source to sink is called the pressure flow hypothesis. According to the "Pressure-Flow Hypothesis", water and dissolved sugars flow through the sieve tubes from an area of higher pressure to an area of lower pressure. The cells in the leaf export sugars into the phloem cells by active transport. The resulting high concentration of sugar causes water to diffuse into the phloem cells, increasing the water pressure there. This area of higher pressure forces the sugar-water solution to move into the next phloem cell. In this manner, sugars are moved from cell to cell.
As the fluid is pushed down (and up) the phloem, sugars are removed by the cortex cells of both stem and root (the "sinks") and consumed or converted into starch. Active transport is necessary to move the sucrose out of the pholem sap and into the cells which will use the sugar -- converting it into energy, starch, or cellulose. As sugars are removed osmotic pressure decreases and water moves out of the phloem. Finally, relatively pure water is left in the phloem, and this leaves by osmosis and/or is drawn back into nearby xylem vessels by the suction of transpiration-pull. Thus it is the pressure gradient between "source" (leaves) and "sink" (refer to areas in the plant that store or use sugars - shoot and roots) that drives the contents of the phloem up and down through the sieve elements.
The movement of sugars in the phloem begins at the source, where (a) sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the movement of water into the dense phloem sap from the neighboring xylem (b). As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins (c), and the sap moves through the phloem. Meanwhile, at the sink (d), incoming sugars are actively transported out of the phloem and removed as complex carbohydrates. The loss of solute produces a high water potential in the phloem, and water passes out (e), returning eventually to the xylem.