Starch is a carbohydrate, built up from simple sugars, which is the main energy storage compound in green plants. Starch provides about 70% of humankind's food in such forms as rice, potatoes, and cereals. Animals and plants convert it to glucose for energy (see respiration).
Starch is a combination of two types of molecules, amylose (normally 20–30%) and amylopectin (normally 70–80%). Both consist of polymers of alpha-D-glucose units in the 4C1 conformation. In amylose these units are linked 1-to-4, with the ring oxygen atoms all on the same side, whereas in amylopectin about one residue in every twenty or so is also linked 1-to-6 forming branch-points. The relative proportions of amylose to amylopectin and 1-to-6 branch-points both depend on the source of the starch, e.g., amylomaizes contain over 50% amylose whereas 'waxy' maize has almost none (~3%).
Starch is used in the manufacture of adhesives and foods (as a thickening agent) and to stiffen laundered clothes and other fabrics.
Compare with cellulose.