Explain a mechanism for the transporting and loading of sugars in the phloem. [6]

Relevant Points:Loading: loading into companion cells, location of companion cells at source(s), active loading of sucrose, mitochondrial action in companion cells, mechanism of proton gradient and co-transport.Transport: mass flow source to sink, high hydro-static pressure at source, inflow of water via osmosis at source, passage through sieve plates, low hydro-static pressure at sink, unloading at sink.
To achieve marks at the higher end students must provide a detailed explanation of both aspects of the question (loading and transportation) and must include references to sources and sinks and the features of different elements on the phloem.