Glycogen is a huge polymer of glucose residues linked by α1-4 glycosidic bonds with branches every 10 residues or so by α1-6 glycosidic bonds. Glycogen gives an significant energy reserve for the body. The two main storage sites are the liver and skeletal muscle where the glycogen is stored as granules in the cytosol. The granules hold not only glycogen but also the enzymes and regulatory proteins which are requires for synthesis and glycogen degraation. Glycogen metabolism is important because it enables the blood glucose level to be maintained by meals (by glycogen stores in the liver) and also provides an energy reserve for muscular activity. The preservation of blood glucose is necessary in order to supply tissues with an easily metabolizable energy source, particularly the brain that uses only glucose except after a long starvation period.