Plant cell walls are mainly composed of cellulose. Cellulose is a different kind of sugar that is classified as a structural carbohydrate that is not used by plant cell for energy. The cellulose protects the organelles that are found inside the plant cell. They serve as a protective layer of the cell and they provide its shape and support. In other plant organisms, they are the secondary walls of the plant cell that increases the rigidity of the cell and they serve as the waterproofing mechanism of plant cells; an example of this are the wood and bark cells of trees.