What is the difference between starch ,glucose and sucrose?

Glucose is a monosaccride. It is a hexose with the chemical formula C6H12O6. It usually appears as a ring form called pyranose. It is a reducing sugar since it has an aldehyde group which possesses reducing properties. when a Benedict's test is conducted on glucose, the blue Benedict solution turns to a mixture containg brick-red precipitate. When glucose solution is added to a clinistix paper, the paper turns from red to blue. It is soluble in water.

Sucrose is a disaccharide. It has a chemical of C12H22O11.It is formed from the reaction(hydrolysis) between alpha-glucose and beta-fructose. It is not a reducing sugar since its redcing groups in the glucose unit and fructose unit is both locked up by the alpha, beta-1,2-glycosidic bond. No change can be observed when sucrose is used to conduct a Benedict's test. It is soluble in water.

Starch is a polysaccride. It is composed of amylose and amylopectin. Amylose is an unbranched polymer chain consists of many glucose monomers linked up by alpha-1,4-glycosidic bond. Amylopectin is branched polymer chains consists of many glucose monomers linked up by alpha-1,4-glycosidic bond and meighbouring chains are linked up by alpha-1,6-glycosidic bond. Starch is a storage form of glucose in plant cells. It is insoluble in water.

2007-09-17 13:37:08 補充：
Presence of starch can be test by iodine test. When iodine solution is added to starch, the iodine solution will turn from brown to dark blue.