A carbohydrate with the empirical formula (CH2
0)n; in organic compounds η is between three and seven. The carbons in a monosaccharide are usually arranged in an unbranched chain with all carbons but one being hydroxylated; the remaining carbon is either ketonic (*ketose
sugars) or aldehydic (aldose
sugars). The commonest monosaccharides are the hexoses, with six carbons, and the pentoses, with five carbons.
All monosaccharides except dihydroxy-acetone are chiral molecules, i.e. they exhibit stereoisomerism. Most naturally occurring monosaccharides are in the D-form, although some L-isomers do occur. In aqueous solution most monosaccharides form a ring structure in which the aldehydic or ketonic carbon links with one of the hydroxylated carbons. Two forms of ring, the *pyranose
and the *furanose
, are commonly seen. The ring structure of a monosaccharide has two isomeric forms, designated α and β.