Glycogen is the stored form of glucose in humans and other vertebrates. It is a highly branched molecule made up of glucose monomers, and is usually stored in liver or muscle cells. Glycogen is the animal equivalent of starch.

Excess sugar must be stored as glycogen to avoid causing osmotic pressure in the cells of animals. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.

The 2D cross sectional view of glycogen is a circle. In the center of the circle is the core protein glycogenin. The protein core looks similar to curly springs and the springs are connected by strings. The glucose polymer chains are spread out from the central core protein like tree branches. Each monomer of glucose is visible in the polymer chain.

Figure 1: 2-D cross-sectional view of glycogen: A core protein of glycogenin is surrounded by branches of glucose units. (Source: Häggström, Mikael. "Medical gallery of Mikael Häggström 2014". Wikiversity Journal of Medicine 1.)