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Translocation of sucrose

Discussion focussing on the mechanism of sucrose translocation between plant tissues through phloem tissue. Sucrose is produced at the 'source' tissue. Companion cells in phloem tissue actively transport protons out of the cell. This creates a proton gradient, which a cotransporter protein uses to transport both protons and sucrose into the companion cells. As the concentration of sucrose rises inside the companion cells it diffuses through the plasmodesma and into the phloem tube. This lowers the water potential inside the phloem tube and causes water to move by osmosis. Hydrostatic pressure rises as a result and causes movement of water and solutes from this region to another which may have a lower hydrostatic pressure, i.e., a sink. Simples!