Monosaccharides are carbohydrates that are the basic units of sugars. These can break down into simpler units of polyhydroxy aldehyde or ketone. Thus, monosaccharides cannot be hydrolysed further. Monosaccharides are classified on the basis of number of carbon atoms and the functional group present in them.

Monosaccharides containing an aldehyde group are known as aldoses and those containing a keto group are known as ketoses. Monosaccharides are further classified as trioses, tetroses, pentoses, hexoses, and heptoses according to the number of carbon atoms they contain. For example, a ketose containing 5 carbon atoms is called ketopentose and an aldose containing 5 carbon atoms is called aldopentose.