the main storage carbohydrate of plants. Starch is formed in cellular organisms in chloroplasts and amyloplasts and is accumulated primarily in seeds, bulbs, and tubers but also in leaves and stems. It is stored away in cells in the form of granules that contain small amounts of proteins and lipids. The starch granules in various plant types differ in size and shape— the largest granules are in the potato (average diameter, approximately 33 μ) and the smallest are in rice (approximately 15 μ)—and have a laminar structure. The origin of starch granules can be determined by examination of their shape under a microscope.
Starch is a mixture of two polysaccharides, linear amyloses and branched amylopectins, which both have the general formula (C6H10O5)n. As a rule, the amylose content in starch is 10-30 percent and the amylopectin content is 70–90 percent. Starch polysaccharides are constructed of glucose residues bound in amylose and in linear amylopectin chains by cross α-l,4-glucoside bonds and at branch points by α-l,6-glucoside bonds. (On the average, approximately 1,000 glucose residues are bound in amyloses, and the individual linear segments of amylopectin molecules consist of 20–30 such units.)
The characteristic blue color of starch produced by an iodine solution (iodine reaction) is used for its identification. Polysaccharides with a reduced degree of polymerization, known as dextrins, are produced in the partial acid hydrolysis of starch. Complete hydrolysis leads to glucose. Enzymatic decomposition of starch may be carried out in various ways. In the presence of inorganic phosphate, plant phosphorylase breaks the α-1,4 bonds with the formation of glucoso-1-phosphate, thus converting starch from its storage form to a metabolically active form.
The enzymes a- and β-amylases, widespread in nature, also break only α-1,4 bonds: β-amylase effects cleavage to maltoses and dextrins, and α-amylase is capable of bypassing the branch points and completely breaking down starch into low-molecular-weight products (for instance, maltose and glucose).
The cleavage of α-1,6 bonds with the formation of free glucose is catalyzed by amylo-l,6-glucosidase. Glucoamylase, an enzyme found in molds, cleaves starch down to glucose. The final products of the enzymatic cleavage of starch are glucose and glucoso-1-phosphate; they are very important substrates for both energetic exchange and biosynthetic processes. The biosynthesis of unbranched starch chains is carried on using glucosyl-trans-ferases that catalyze the transfer of the glucose residue from nucleoside diphosphate glucose to the growing carbohydrate chain. The “branching” Q enzyme transfers the end glucose residue from the major chain to a side chain with the formation of the α-1,6 bond in amylopectins. The starting substrate in starch biosynthesis in plants may be saccharose.
Starch is a basic component of the most important food products. For example, starch makes up 75–80 percent of flour and 25 percent of potatoes. It is easily digested in the gastrointestinal tract and has a high caloric content: 16.75 kilojoules per g (approximately 4 kilocalories per g). Starch and its products are used in the production of paper, textiles, and glues, as well as in foundry casting and other industrial fields.
Starch is added to powders, salves, and pastes. It is used in 1-percent solutions as an indicator of iodine. Starch is also used as a coating (sizing and boiled starch). Capsules are prepared from a mixture of starch (or wheat flour) and starch paste.
D. M. BELEN’KII