Abstract
Sucrose and starch are the major end products of photosynthesis. Sucrose is actively loaded into the phloem and exported to those parts of the plant having a higher demand for carbohydrate (the sink tissues) whilst starch is accumulated in the chloroplasts of the mature leaves (the source tissues). Depending on the needs of the plant, the starch can be degraded to glucose-1-phosphate and finally leave the chloroplast as triose phosphate, which after conversion to sucrose in the cytoplasm, can also be loaded into the phloem by the sucrose carrier. There is evidence that the uptake of sugar into the cells of the sink tissues also occurs either via a sucrose carrier or, after degradation of sucrose to glucose and fructose, by cell wall invertases, via a monosaccharide carrier located in the plasmalemma of the respective cells. It is generally accepted that all of these plant plasma membrane sugar transporters are driven by the proton motive force.
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