What is hard and made of cellulose in a plant cell?
A cell wall
A cellulose sponge is made of, well, cellulose. Cellulose is the main component of plant cell walls, so many plant derived products are cellulose based. Wood is about 50% cellulose, paper and cardboard are almost entirely cellulose, and most plant fibers are mostly cellulose- cotton is about 90% cellulose. the wikipedia article is decent: http://en.wikipedia.org/wiki/Cellulose