Phloem: The pressure-flow mechanism (or Source to Sink) is a model for phloem transport now widely accepted.The model has the following steps.Step 1: Sugar is loaded into the phloem tube from the sugar source, e.g. the leaf (active transport)Step 2: Water enters by osmosis due to a high solute concentration in the phloem tube. Water pressure is now raised at this end of the tube.Step 3: At the sugar sink, where sugar is taken to be used or stored, it leaves the phloem tube. Water follows the sugar, leaving by osmosis and thus the water pressure in the tube drops.The building up of pressure at the source end, and the reduction of pressure at the sink end, causes water to flow from source to sink. As sugar is dissolved in the water, it flows at the same rate as the water. Sieve tubes between phloem cells allow the movement of the phloem sap to continue relatively unimpeded.