Glycogen

Glycogen is a polysaccharide, a long chain of glucose units, and the main way animals keep sugar on hand for quick use; fungi and many bacteria store it too. Its glucose units are joined mostly by alpha 1-4 bonds into chains, with alpha 1-6 bonds starting a new branch about every ten units. It resembles amylopectin, the branched form of plant starch, but is more heavily branched. The branching makes glycogen compact and gives it many chain ends where enzymes can add or remove glucose at once.

Most glycogen sits in the liver and in skeletal muscle, and the two stores serve different purposes. After a meal, insulin promotes glycogen synthesis. When blood glucose falls, hormones such as glucagon and adrenaline trigger its breakdown, called glycogenolysis. The liver has the enzyme glucose-6-phosphatase, so it can release free glucose into the blood and keep the brain supplied between meals. Muscle lacks this enzyme and keeps its glucose for its own use during exercise. Liver glycogen runs low within a day or so of fasting, after which gluconeogenesis supplies most of the blood's glucose.

Inherited defects in the enzymes that build or break down glycogen cause glycogen storage diseases, such as von Gierke disease and Pompe disease, which can bring low blood sugar, an enlarged liver or muscle weakness.