Describe the mass flow hypothesis of translocation.

Photosynthesis occurs in the leaf (source), making sugars: fructose and glucose. These are converted to sucrose and are actively transported into the phloem (sieve tube element) by companion cells. The increase of sugars lowers the water potential in the phloem and therefore draws in water down a concentration gradient by osmosis. This increase in water causes an increase in pressure within the phloem which causes a mass movement towards the roots. Sugars then move down a concentration gradient into the sink where they are stored or used for respiration.