Final answer:
Glycogen is a branched polysaccharide that serves as an energy storage molecule in animals, synthesized and stored in liver and muscle cells, and rapidly converted into glucose during energy demands.
Step-by-step explanation:
Glycogen is a highly branched polysaccharide used for energy storage by animals, including humans. This molecule is composed of glucose monomers and is similar in structure to amylopectin, but with more frequent branching which allows for quick access to glucose units during energy needs. Glycogen is synthesized and stored predominantly in the liver and muscle cells. When blood sugar levels decrease, a process known as glycogenolysis occurs where glycogen is broken down into glucose, which can then be released into the bloodstream to maintain normal blood sugar levels and provide energy for cellular processes, particularly during times of fasting or intense exercise.