Two main functions of polysaccharides in living things are:
1. structural components
2. energy storage.
Those polysaccharides that are not easily broken down are used as structural components. Some of them are cellulose in plants and chitin in insects and crustacean.
On the other hand, starch and glycogen are easily broken down, so they serve as energy storage. When energy is needed immediately, they break down to glucose and great amount of energy is released,