Source and Sink

Every phloem translocation pathway connects a source to a sink. A source is any organ capable of exporting organic food to areas of metabolism or storage elsewhere in the plant - typically a mature, actively photosynthesising leaf, though a germinating seed mobilising its own stored reserves also functions as a source. A sink, by contrast, is any organ that only ever receives food from a source and never exports it - common examples include roots, storage tubers, developing fruits, and young, immature leaves that have not yet matured into net exporters themselves. A related and genuinely interesting question is why plants specifically move their food as sucrose, rather than as glucose, fructose, or starch. Glucose and fructose are simple monosaccharides that would be an inefficient form in which to move large amounts of stored energy; starch, while an efficient way to store energy, is insoluble in water and therefore cannot travel through the phloem's watery sap at all. Sucrose, a disaccharide built from one glucose and one fructose unit, solves both problems at once - it is water-soluble, so it can actually move through the sieve tubes, it stores energy efficiently like starc …