Monosaccharides glucose (a hexose) a fructose (a pentose), shown in both open-chain (left) and hemiacetal-ring (right) forms.

Monosaccharides are simple sugars having the general formula (CH2O)n. They are classified by the number of carbon atoms in each of their molecules: trioses (3), tetroses (4), pentoses (5), hexoses (6), and so on (up to 9). The most abundant natural monosaccharide are the hexoses, C6H12O6 (including glucose), and the pentoses (including xylose). Many different isomers of these sugars are possible and often have names reflecting their source, or property, e.g., fructose is formed in fruit, arabinose in gum arabic, and the pentose, xylose, in wool.


Monosaccharides are sweet-tasting, cannot be broken down by hydrolysis, and combine to form more complex sugars known as disaccharides, oligosaccharides, and polysaccharides.