The pathway of starch synthesis is known to involve the inter-conversion of sugars, sugar-phosphates and nucleotide-sugars. Amylopectin is synthesized by Starch Synthases and branched by Branching Enzyme. Amylose is synthesized by Granule-Bound Starch Synthase.
ADP-glucose is synthesized by the enzyme ADP-glucose pyrophosphorylase, previously believed to be restricted to the plastid compartment of the cell. Very recently it has been found that pyrophosphorylase may be located in the amyloplast in potato and peas, but in corn and barley there is evidence which suggests that this enzyme may be located in the cytoplasm. It can be speculated that cereals in general have evolved uniquely away from their primordial origins. The exact compartmentation of ADP-glucose pyrophosphorylase remains controversial but provides an example of one important area of future research efforts.