Starch and glycogen are two types of carbohydrates, or polysaccharides, that function as energy storage molecules. In plants, starch is the primary form of energy storage, whereas glycogen is the primary form of energy storage in animals. Despite their similar roles in energy storage, starch and glycogen have distinct structural features.
Starch is composed of two polysaccharides, amylose and amylopectin. Amylose is a linear polysaccharide composed of D-glucose molecules joined by α-1,4 glycosidic bonds. Amylopectin is a branched polysaccharide composed of D-glucose molecules joined by α-1,4 glycosidic bonds, as well as α-1,6 glycosidic bonds. These α-1,6 glycosidic bonds are responsible for the branched structure of amylopectin.
Glycogen, on the other hand, is composed of only one polysaccharide, amylopectin. Like amylopectin, glycogen is highly branched, with α-1,4 and α-1,6 glycosidic bonds. However, glycogen has an even higher degree of branching than amylopectin. This increased branching makes glycogen particularly well-suited for rapid energy release since its numerous branches provide numerous sites for hydrolysis.
Another major difference between starch and glycogen is their solubility in water. Starch is insoluble in water, whereas glycogen is highly water-soluble. This property of glycogen is important for animals, since it allows the molecule to be stored in aqueous solutions in the cell.
In summary, the major structural difference between starch and glycogen is the degree of branched structure. Starch is composed of two polysaccharides, amylose and amylopectin, whereas glycogen is composed of only one polysaccharide, amylopectin. Additionally, starch is insoluble in water, whereas glycogen is highly water-soluble. These differences in structure and solubility are important for the efficient storage and release of energy in plants and animals, respectively.
Starch
Starch is a polysaccharide consisting of glucose monomers that are joined together by alpha-1,4 and alpha-1,6 glycosidic bonds. Starch is found in plants, where it is primarily responsible for energy storage. It can be found in the form of amylose and amylopectin, with amylose making up 20-30% and amylopectin making up 70-80%. Amylose is a linear chain of glucose molecules, while amylopectin is a branched chain of glucose molecules. When starch is digested, it is broken down into simple sugars such as glucose, maltose, and maltotriose.
Glycogen
Glycogen is a polysaccharide consisting of glucose monomers that are linked together by alpha-1,4 glycosidic bonds and alpha-1,6 glycosidic bonds. Like starch, glycogen is primarily responsible for energy storage. Glycogen is the storage form of glucose in animals and is mainly found in the liver and muscles. It is composed of two components, the outer layer which consists of short alpha-1,4 glycosidic bonds, and the inner layer which consists of longer alpha-1,6 glycosidic bonds. Glycogen is broken down into simple sugars such as glucose, maltose, and maltotriose when it is digested.
Major Structural Difference Between Starch and Glycogen
The major structural difference between starch and glycogen is the length of the alpha-1,6 glycosidic bonds. In starch, the alpha-1,6 glycosidic bonds make up a small portion of the molecule (20-30%) and are relatively short, while in glycogen the alpha-1,6 glycosidic bonds make up a larger portion of the molecule (70-80%) and are much longer. This difference in length of the alpha-1,6 glycosidic bonds makes it easier for glycogen to be broken down into simple sugars when it is digested, allowing it to be used more quickly for energy. Additionally, the branching of glycogen allows it to store more glucose molecules in the same space as starch, making it more efficient at storing energy.
