Phloem transport - pressure flow hypothesis

Companion cells are closely associated with sieve tube elements and help maintain the pressure gradient required for translocation of food (loading and unloading of sugars) through the sieve tubes.

Phloem loading is the movement of sugars (mainly sucrose) from the mesophyll cells (source) into the sieve tubes of the phloem, mostly by active transport.
This increases the sugar concentration in the phloem, so water enters the sieve tubes by osmosis and builds up pressure, which drives the mass flow of sap towards the sinks (Münch's pressure flow hypothesis).
So phloem loading is related to the increase of sugar in the phloem.