(amylum) The most abundant and important reserve polysaccharide in plants. Starch is an early end product of the photosynthetic reduction of carbon dioxide in chloroplasts: in the dark the starch thus formed is rapidly broken down to sucrose and transported to other organs.
The bulk of the starch in plants is found in storage organs. Commercial extraction of starch is from such organs. Common sources are the roots of cassava (Manihot esculenta), which yield tapioca, the rhizomes of arrowroot (Maranta arundinacea), the stem pith of the sago palms (species of Metroxylon, Arenga, and certain other genera), the tubers of potato, and the grains of various cereals, especially maize, wheat, rice, and sorghum.
Starch consists of two structurally different fractions, *amylose
. The relative amounts of amylose and amylopectin in a given starch sample depend on the species of plant from which the starch was obtained, amylose usually making up 20-30% of the starch granules. The shape of the starch grains formed by different species also differ (see hilum) and this variation has been used in taxonomic work. Starch is synthesized from ADP-glucose by the enzyme starch synthetase. A branching enzyme, known as Q enzyme, is responsible for formation of the α(1 -6) bonds in amylopectin. Starch breakdown is catalysed by a class of enzymes known as amylases. These enzymes break down starch to the disaccharide *maltose
, which is then further degraded by the disaccharidase maltase. Amylase activity is high during periods of rapid growth such as seed germination or sprouting of tubers.