=Structural Isomerism.=--This refers to an actual difference in the characteristic groups which are present in the molecule. As has been said, all carbohydrates, from the standpoint of the characteristic groups which they contain, are aldehyde-alcohols. The hexoses all contain five alcoholic groups and one primary aldehyde, or one secondary aldehyde (ketone), group. If the aldehyde oxygen is attached to the carbon atom which is at the end of the six-membered chain, the structural arrangement is | that of an aldehyde, C=O and the sugar is of the type known | H as "aldoses"; whereas, if the oxygen is attached to any other | carbon in the chain, the ketone arrangement, C=O results and | the sugar is a "ketose." This difference is illustrated in the Fischer open-chain formulas for glucose (an aldose) and fructose (a ketose) as follows: Glucose Fructose CH_{2}OH CH_{2}OH | | CHOH CHOH | | CHOH CHOH | | CHOH CHOH | | CHOH C=O | | CHO CH_{2}OH =Stereo-isomerism=, or space isomerism, as its name indicates, depends upon the different arrangement of the atoms or groups in the molecule in space, and not upon any difference in the character of the constituent groups. This possibility depends upon the existence in the molecule of the substance in question of one or more _asymmetric carbon atoms_ and manifests itself in differences in the optical activity of the compound.[1] Thus, in the formula for glucose shown above there appear four asymmetric carbon atoms, namely, those of the four secondary alcohol groups (in the terminal, or primary alcohol, group, carbon is united to hydrogen by two bonds, and in the aldehyde group it is united to oxygen by two bonds). Similarly, fructose contains three asymmetric carbon atoms. As an example of how the presence of these asymmetric carbon atoms results in the possibility of many different space relationships, the following graphic illustrations of the supposed differences between dextro-glucose and levo-glucose, and between dextro- and levo-galactose, may be cited.[2] _d_-glucose _l_-glucose _d_-galactose _l_-galactose