A carbohydrate is a simple sugar or a molecule composed of two or more simple sugars. In general, the ratio of carbon, hydrogen, and oxygen atoms is 1:2:1 in a carbohydrate molecule. There are three classes of carbohydrates: monosaccharides, oligosaccharides, and polysaccharides. Glucose, sucrose, starch, and cellulose are examples of carbohydrates. In all living organisms, carbohydrates are broken down to provide usable chemical energy for cells. In plants, the carbohydrate cellulose is used for structural support in making cell walls.