They have a general formula (C H₂O) n where n equal to N° of carbon atoms (3-7). They are the simplest basic units of carbohydrates. They usually contain 3, 4, 5, 6, or 7 carbon atom. They either have aldehydes (CHO) or Ketone (C=O) functional group. Those with the aldehyde functional group are known as the aldo sugar (aldoses) e.g.: glucose while those with the Ketone functional group are known as Ketosugars (Ketoses) e.g.: fructose.

Generally the three carbon sugars are known as trioses, 4 tetroses, 5 pentoses, 6hexoses, 7 heptoses. The two simplest Monosaccharide are the trioses e.g.: glyceraldehydes and dihydroxoacetone. Glyceraldehydes have the aldehyde functional group while dihydroxoacetone has the Ketone functional group.

Of all the Monosaccharide, the pentoses and the hexoses are the most common.  Monosaccharides are the most important energy sources and are the building blocks for the synthesis of larger molecules.

PHYSICAL PROPERTIES OF MONOSACCHARIDES

1-They are small molecules, that is monomers of two molecular weight.

2-They are very soluble in H₂O

3-They are crystalline, that is can be crystallised

4-They are sweet

NOTE: The most abundant monosaccharide is the six carbon sugars called hexoses with molecular formula C₆H₁₂O₆   

At any one time, must molecules exist as rings rather than chains. In the case of glucose, carbon atom number 1 combines with the O₂ of carbon atom 5. This forms a six sided structure called Pyranose ring. In the case of fructose, it is carbon atom number II which links with the O₂ of carbon atom five to form a five sided structure called Furanose ring.