Carbohydrates come from a wide array of foods – bread, beans, milk, rice and etc. They also come in a variety of forms. The most common and abundant are sugars, fibers, and starches. The basic building block of a carbohydrate is a sugar molecule, a simple union of carbon, hydrogen, and oxygen. Starches and fibers are essentially chains of sugar molecules. Some contains hundreds of sugars. Some chains are straight, others branch wildly.
The basic carbohydrate units are called monosaccharide, such as glucose, galactose, and fructose. The general chemical formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater. Monosaccharide can be linked together in almost limitless ways. Two joined monosaccharide are called disaccharides, such as sucrose and lactose. Carbohydrates containing between about three to six monosaccharide units are termed oligosaccharides; anything larger than this is a polysaccharide. Polysaccharides, such as starch, glycogen, or cellulose, can reach many thousands of units in length. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed.
Starch is the major form of stored carbohydrate in plant cells. Its structure is identical to glycogen, except for a much lower degree of branching (about every 20-30 residues). Unbranched starch is called amylose; branched starch is called amylopectin.
Amylose consists of linear, unbranched chains of several hundred glucose residues (units). The glucose residues are linked by a glycosidic bond between their #1 and #4 carbon atoms. While amylopectin differs from amylose in being highly branched. At approximately every thirtieth residue along the chain, a short side chain is attached by a glycosidic bond to the #6 carbon atom (the carbon above the ring). The total number of glucose residues in a molecule of amylopectin is several thousand.
Starches are insoluble in water and thus can serve as storage depots of glucose. Plants convert excess glucose into starch for storage. Rice, wheat, and corn are some major sources of starch in the human diet.