Transport of sugar in plants

Phloem is composed of living cells that transport a water solution of sugars...This movement is modeled by the pressure-flow theory, a part of which is that the sugar-containing fluid is moved through sieve tubes by fluid pressure. By this means, nutrients can be moved from the photosynthetic site to the place where the sugar is being used whether it is up or down the stem of the plant.

Part of the pressure-flow theory is that the sucrose produced is moved by active transport into the companion cells of the phloem in leaf veins. This raises the concentration of sucrose molecules in the companion cells above that in the sieve tubes, so they can then move into the sieve tubes by diffusion. With the concentration of sucrose now greater in the sieve tubes than external to them, water molecules will move into the sieve tubes near those photosynthesis locations by osmosis. With a larger amount of water in the tube, its fluid pressure will be higher than at distant locations in the tube, and the pressure difference will cause flow in those directions.

At some distance from the photosynthetic source, there may be a region, say in a fruit, where sugar is needed. This theory suggests that the sucrose is transported into the fruit by active transport, raising the sugar concentration in the fruit relative to the sieve tube. In response to this concentration difference, water will follow the sugar into the fruit by osmosis.

http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/plaene.html