<P> Two monosaccharides with equivalent molecular graphs (same chain length and same carbonyl position) may still be distinct stereoisomers, whose molecules differ in the three - dimensional arrangement of the bonds of certain atoms . This happens only if the molecule contains a stereogenic center, specifically a carbon atom that is chiral (connected to four distinct molecular sub-structures). Those four bonds can have any of two configurations in space distinguished by their handedness . In a simple open - chain monosaccharide, every carbon is chiral except the first and the last atoms of the chain, and (in ketoses) the carbon with the keto group . </P> <P> For example, the triketose H (CHOH) (C = O) (CHOH) H (glycerone, dihydroxyacetone) has no stereogenic center, and therefore exists as a single stereoisomer . The other triose, the aldose H (C = O) (CHOH) H (glyceraldehyde), has one chiral carbon--the central one, number 2--which is bonded to groups − H, − OH, − C (OH) H, and − (C = O) H. Therefore, it exists as two stereoisomers whose molecules are mirror images of each other (like a left and a right glove). Monosaccharides with four or more carbons may contain multiple chiral carbons, so they typically have more than two stereoisomers . The number of distinct stereoisomers with the same diagram is bounded by 2, where c is the total number of chiral carbons . </P> <P> The Fischer projection is a systematic way of drawing the skeletal formula of an acyclic monosaccharide so that the handedness of each chiral carbon is well specified . Each stereoisomer of a simple open - chain monosaccharide can be identified by the positions (right or left) in the Fischer diagram of the chiral hydroxyls (the hydroxyls attached to the chiral carbons). </P> <P> Most stereoisomers are themselves chiral (distinct from their mirror images). In the Fischer projection, two mirror - image isomers differ by having the positions of all chiral hydroxyls reversed right - to - left . Mirror - image isomers are chemically identical in non-chiral environments, but usually have very different biochemical properties and occurrences in nature . </P>

The most numerous functional group to be found in a monosaccharide is