Plant cells manufacture glucose through photosynthesis. When glucose is present in excess, plants store it by using it to synthesize chains of sugar molecules called starches. These starches form an important component of the human diet.
Each molecule of glucose has a hexagonal ring structure and contains six carbons. Starches are very long chains of glucose molecules formed by bonds called glycosidic bonds between carbon 1 of one glucose molecule and carbon 4 of the next. When glucose molecules are bonded together, a water molecule is removed as a product of the reaction. This type of process is called a dehydration reaction.
There are two types of starches: amylose and amylopectin. Both are similar in structure, but amylose is linear and amylopectin is branched. Plants store these starches in granules called plastids inside plant cells.
Starches are similar in some respects to another polymer called cellulose; both starch and cellulose are long chains of glucose molecules. In cellulose, however, the glucose molecules are in a slightly different configuration called beta-glucose, so when they form bonds, each glucose molecule is upside-down when compared with its neighbors. This difference enables the chains in cellulose to hydrogen-bond with each other, forming tough rope-like fibers that act as the main structural component of plant cell walls.