Question

what is glycogen n how it is usefull

Answer 1

in humans, glycogen is made and stored in liver and muscle cells. Muscle cell glycogen is broken down into glucose, and liver glycogen is broken down into glucose as a circulating energy source glucose for use by the body. Glycogen is an energy reserve that can be quickly mobilized to meet sudden needs.

Answer 2

glycogen is food storage molecule in animals. Its significance is to store food material and convert it to glycose when energy is required.
Glycogen is a polysaccharide with branched structure.
hope it helps