<P> A carbohydrate is a biological molecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen--oxygen atom ratio of 2: 1 (as in water); in other words, with the empirical formula C (H O) (where m could be different from n). This formula holds true for monosaccharides . Some exceptions exist; for example, deoxyribose, a sugar component of DNA, has the empirical formula C H O. Carbohydrates are technically hydrates of carbon; structurally it is more accurate to view them as polyhydroxy aldehydes and ketones . </P> <P> The term is most common in biochemistry, where it is a synonym of' saccharide', a group that includes sugars, starch, and cellulose . The saccharides are divided into four chemical groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides . Monosaccharides and disaccharides, the smallest (lower molecular weight) carbohydrates, are commonly referred to as sugars . The word saccharide comes from the Greek word σάκχαρον (sákkharon), meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides very often end in the suffix - ose . For example, grape sugar is the monosaccharide glucose, cane sugar is the disaccharide sucrose, and milk sugar is the disaccharide lactose . </P>

List by symbol the types of atoms that can be found in carbohydrates