Glycogen Granules

Glucose is the favored "fuel molecule" of the brain. To protect the brain from a possible fuel shortage, the body attempts to maintain a constant level of glucose in the blood. Excess dietary glucose is therefore polymerized to the polysaccharide glycogen that is stored in liver and muscle cells. When the level of glucose in the blood begins to drop, this reservoir of glucose can quickly be mobilized.

The primary structure of glycogen resembles that of amylopectin but glycogen is more highly branched, with branch points occurring every 8 to 12 glucose deposits.

Glycogen is kept in cytoplasmic granules of muscle and liver cells that contain up to 120,000 glucose units. Glycogen granules also contain the enzymes that catalyze glycogen synthesis and degradation as well as some of the enzymes that moderates these processes. Compared to fat, glycogen has several advantages as a short-term energy reservoir: Muscles can activate the energy stored in the glucose units of glycogen much faster than they can activate the energy stored in fat. Glucose, in contrast to fatty acids, can be metabolized in the absence of free oxygen and thereby provides a very fast pathway of energy generation. Humans and animals are also not able to convert fatty acids to glucose. Therefore, fat metabolism alone is not adequate to maintain blood glucose levels.