A carbohydrate is a sugar, or one of its dimers, or polymers (monosaccharides, disaccharides, and polysaccharides). Carbohydrates serve as fuel and building material. Cellulose is a polysaccharide that is a major component of the tough walls that enclose plant cells. Cellulose is a polymer of glucose. Most fresh fruits (like the apple above), vegetables, and whole grains are abundant in cellulose.