SYNTHESIS OF STARCH AND SUCROSE In most species, sucrose is the principal form of carbohydrate translocated throughout the plant by the phloem. Starch is an insoluble stable carbohydrate reserve that is present in almost all plants. Both starch and sucrose are synthesized from the triose phosphate that is generated by the Calvin cycle. The pathways for the synthesis of starch and sucrose are shown in below figure. Starch Is Synthesized in the Chloroplast The prominent starch deposits, as well as enzyme localization studies about the chloroplast , leave no doubt that the chloroplast is the site of starch synthesis in leaves. Starch is synthesized from triose phosphate via fructose-1,6-bisphosphate. The glucose-1-phosphate intermediate is converted to ADP-glucose via ADP-glucose pyrophosphorylase in a reaction that requires ATP and generates pyrophosphate (PPi, or H 2 P 2 O 7 2– ). As in many biosynthetic reactions, the pyrophosphate is hydrolyzed via a specific inorganic pyrop...
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