Chiral carbons (carbons covalently linked to 4 different entities) give rise to stereoisomers. Molecules that are stereoisomers have the same formula and the same structure, but have their atoms arranged in different ways in 3D space. For example, compare the structures of D-glyceraldehyde and L-glyceraldehyde in Figure 9.5. Notice that they are nonsuperimposable.
Common sugars typically have not one, but multiple chiral carbons. Glucose, for example, contains four chiral carbons. For a carbon with 'm' chiral carbons, the number of possible stereoisomers is 2m. Thus, for glucose, there are 16 possible stereoisomers. The form most commonly found in living organisms, D-glucose, has only one mirror image. In fact, any stereoisomer has only one mirror image. The other 14 stereoisomers of glucose that are not mirror images of it are called diastereomers. That is, diastereomers are stereoisomers that are not mirror images of each other.
Diastereomers can have very different biological uses. For example, Figure 9.9a shows the 8 D-stereoisomers of D-glucose. Some of these, such as galactose and mannose, are commonly found in biological systems. Others, such as D-idose or D-talose, are much rarer.