Chitin and cellulose are both polysaccharides. Chitin is sturctural form of glucose in insects, the exoskeleton. Cellulose is the structural form of glucose in plants. When you "snap" a piece of plant, the "snap" is the cellulose. Cellulose can not be broken down by the stomache. It gets pushed out through the excretory system. Cellulose is fiber.