Transport of Sugars

Translocation, the movement of carbohydrates from their site of production to where they are needed, is accomplished through the pressure-flow theory. In this theory, sugar enters the sieve tube cells of the phloem, reducing the concentration of water inside. This causes water to osmose into the sieve tube in an attempt to balance the concentration. Meanwhile, where cells are using sugar, the concentration of sugar is falling. This leads to a concentration gradient from the leaves (where sugar production occurs) to the site where sugar is needed. Because the sieve tube cell walls are rigid, the entry of excess water causes pressure to rise. This causes the water in the phloem by the leaves to push away in an attempt to equalize the pressure -- at the same time carrying sugar with it.