Monosaccharides are the simplest carbohydrates since they cannot be hydrolyzed into smaller carbohydrates. Chemically they are aldehydes or ketones possessing two or more hydroxyl groups, and are important as building blocks (such as D-ribose and 2-deoxy-D-ribose) for the synthesis of nucleic acids, as well as fuel molecules in glycolysis. The most common naturally occurring monosaccharides are D-glucose, D-fructose, D-mannose, and D-galactose among the hexoses and D-xylose and L-arabinose among the pentoses. Monosaccharides are classified according to three different characteristics: the location of their carbonyl group, the number of carbon atoms they contain, and their chiral property. If the carbonyl group is an aldehyde, the monosaccharide is an aldose. If the carbonyl group is a ketone, the monosaccharide is a ketose. Monosaccharides with three carbon atoms are called trioses and are the smallest monosaccharides, and four carbon atoms for tetroses, five carbons for pentoses, six carbons for hexoses, and so on.

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