The diversity in starch granule structure and properties is appalling. Even cereal starches widely vary in properties. Starch occurs in intracellular granules and does not have any function as structure-building molecule in plant cells, but rather functions as a storage polysaccharide.
The word of starch comes from Middle English word, strechen meaning ‘to stiffen’. Starch is produced by plant in the form of granules mainly containing two biopolymers: amylose and amylopectin.
The amylose molecule is a linear, unbranched structure in which the glucose residues are made up of α-(1-4)-d linked polyglucan in a helix conformation.
Amylopectin, on the other hand, is a branched-chain polymer, the branch points occurring through a-1,6 glycosidic bonds. In amylopectin about 5% of linkage consists of α(1-6) bonds resulting in a large multi-branched macromolecule with branch-on-branch structure and a molecular size ranging between 50 and 500x106, one of the largest polymers present in nature.
Plants normally store starch with a small amount of water, lipids and proteins in semi-crystalline granules inside amyloplasts and chloroplasts.
Starch granules are insoluble in water and alcohol but can be dissolved in hot water during a process known as gelatinization. During gelatinization, the starch granules are irreversibly destroyed releasing the amylose and amylopectin biopolymers.
The temperature at which gelatinization process takes place is around 70 ◦C for most starch granules, undergoing an irreversible transition in which the molecular structure of amylose and amylopectin is disrupted.
Starch provides humans with energy 4 kcal per gram and is hydrolyzed to glucose, supplying the glucoses that is necessary for brain and central nervous system functioning.
To digest starches, humans have specialized digestive enzymes called amylases. These enzymes breakdown starches into glucose, which is either used immediately as an energy source or stored in the liver and to be released when needed.
Starch: Plant polysaccharide
