Chloroplast, starch storage

During very active periods of photosynthesis, triose phosphates are converted to starch. Under normal conditions, approximately 30% of the CO, fixed by leaves is incorporated into starch, which is stored as water-insoluble granules. During a subsequent dark period, most chloroplast starch is degraded and converted to sucrose. Sucrose is then exported to storage organs and rapidly growing tissues. In these tissues (e.g, tubers and seeds), most sucrose molecules are used to synthesize starch, which is stored primarily within a specialized plastid called an amyloplast. 

Starch, a reserve polysaccharide widely distributed in plants, is the most important carbohydrate in the human diet. In plants, starch is present in the chloroplasts in leaves, as well as in fruits, seeds, and tubers. The starch content is especially high in cereal grains (up to 75% of the dry weight), potato tubers (approximately 65%), and in other plant storage organs. 

Although native starch granules from storage organs seem to be first eroded by amylases before other enzymes can further hydrolyze it, the chloroplastic phosphorylase can release labeled glucose 1-phosphate from 14C-labeled starch granules, at least in pea leaves (Kruger and ap Rees, 1983). 

Starch is bio synthesized in plant organelles. In leaf or transitory starch, the organelles are chloroplasts and in storage starch in seeds, tubers, stems, and roots, the organelles are amylo-plasts. The process of starch biosynthesis is divided, for convenience of discussion, into three steps (1) initiation, (2) elongation, and (3) termination, and for amylopectin, a fourth step, branching. 

The dark reaction (Calvin cycle) uses the NADPH and ATP to make glyceraldehyde 3-phosphate (triose phosphate), which is metabolized initially to starch, sucrose, and cellulose. Starch and sucrose are the major plant storage products. Starch is synthesized from ADP-glucose in the chloroplast, sucrose from fructose 6-phosphate and UDP-glucose in the leaf cytosol. 

The primary function of starch is to serve as energy storage and as a carbon source for de novo biosynthesis of macromolecules. Starch can accumulate temporarily in the chloroplast of cells found in photosynthetic tissue. Most starch is found in the storage organs such as the endosperm of seeds or in roots and tubers. 

Following fixation of the CO2 by ribulose bisphos-phate carboxylase (EC 4.1.1.39) (see) u-fructose-6-phosphate (F-6-P) can be tapped off the Calvin cycle (as photosynthetic product) and used for the synthesis of storage glucan, e.g. starch, within the chloroplast. Additionally dihydroxyacetone phosphate can be tapped off the cycle and exported from the chloroplast to the cytoplasm where it can be used to synthesize sucrose (see Fig. 1 of the entry Calvin cycle). The sucrose is then exported from the cell to the growing parts of the plant. 

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