Carbohydrate. Any member of a broad class of substances which, with the exception of glycogen (q.v.), are of plant origin and are made up of carbon, hydrogen and oxygen, the ratio of the last two elements being nearly always the same as in water. They are conveniently classified according to their complexity into monosaccharides (q.v.), oligosaccharides (q.v.), and polysaccharides (q.v.). The first group represents essentially the building blocks for the other categories and includes the simplest sugars; the second group comprises polymers (q.v.) made up of only a few monosaccharides, while the polysaccharides consist of hundreds and even thousands of monosaccharides and comprise the dextrins (q.v.), starches (q.v), cellulose (q.v), plant gums (q.v.) and other highly complex polymers.