Starch
Energy store of plants

Plants produce glucose from carbon dioxide and water by photosynthesis. The glucose is used to generate the chemical energy required for general metabolism as well as a precursor to myriad organic building blocks such as nucleic acids, lipids, proteins, and structural polysaccharides such as cellulose. Most green plants store any extra glucose in the form of starch, which is packed into semicrystalline granules called starch or amyloplasts. Toward the end of the growing season, starch accumulates in twigs of trees near the buds. Fruit, seeds, rhizomes, and tubers store starch to prepare for the next growing season. Young plants live on this stored energy in their roots, seeds, and fruits until they can find suitable soil in which to grow. The starch is also consumed at night when photosynthesis is not occurring.

Green algae and land-plants store their starch in the plastids, whereas red algae, glaucophytes, cryptomonads, dinoflagellates and the parasitic apicomplexa store a similar type of polysaccharide called floridean starch in their cytosol or periplast.

Especially when hydrated, glucose takes up much space and is osmotically active. Starch, on the other hand, being insoluble and therefore osmotically inactive, can be stored much more compactly. The semicrystalline granules generally consist of concentric layers of amylose and amylopectin which can be made bioavailable upon cellular demand in the plant.

Amylose consists of long chains derived from glucose molecules connected by α-1,4- glycosidic linkage. Amylopectin is highly branched but also derived from glucose interconnected by α-1,6- glycosidic linkages. The same type of linkage is found in the animal reserve polysaccharide glycogen. By contrast, many structural polysaccharides such as chitin, cellulose, and peptidoglycan are linked by β-glycosidic bonds, which are more resistant to hydrolysis.
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