Any of the simple carbohydrates .
They are classified based on their backbone of carbon (C) atoms: Trioses have three carbon atoms, tetroses four, pentoses five, hexoses six, and heptoses seven. The carbon atoms are bonded to hydrogen atoms (―H), hydroxyl groups (―OH; see functional group ), and carbonyl groups (―C=O), whose combinations, order, and configurations allow a large number of stereoisomers (see isomer ) to exist. Pentoses include xylose, found in woody materials; arabinose, found in gums from conifers; ribose, a component of RNA and several vitamins ; and deoxyribose, a component of DNA . Important hexoses include glucose , galactose , and fructose . Monosaccharides combine with each other and other groups to form a variety of disaccharides, {{link=polysaccharide">polysaccharides , and other carbohydrates.