How does translocation occur in the phloem?

Active transport occurs at the source, loading assimilates e.g. from photosynthesis in a leaf. This is carried out by specialised companion cells, via active pumping of hydrogen ions out of the cell, and cotransporter proteins allowing only hydrogen and sucrose molecules to enter the cell together, down a electrochemical and pH gradient. Companion cells are connected to seive tube elements via plasmodesmata, and allows constant transport of sucrose into the phloem. Increased concentrations of solutes in the area causes water to flow in via osmosis. This raises the hydrostatic pressure at the source, and mass flow of liquid (carrying solutes) away from the source due to a pressure gradient allows assimilates to be translocated to sinks, where there is a lower hydrostatic pressure due to active unloading of solutes (and therefore water also following via osmosis).