phloem unloading

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The processes by which sugars and other substances leave the conducting vessels of the phloem and are delivered to their destination (the sink) in the plant. These processes mirror those responsible for phloem loading in that both a symplastic pathway, involving intracellular diffusion via plasmodesmata, and an apoplastic pathway, via the intercellular space, are used variously, depending on the type of tissue and species of plant. Apoplastic unloading may have two possible routes. In one the sucrose diffuses from the phloem companion cells into the intercellular space, where it is split by an invertase enzyme into its component monosaccharides (glucose and fructose), which are actively taken up by the sink cells. In the other route the sucrose is actively pumped out of companion cells and subsequently diffuses into the sink cells.

Subjects: Biological Sciences.

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