ointed out later (see Chapter XIV), all chemical reactions
which are involved in vital phenomena, including those of plant growth and
metabolism, are controlled by enzymes. The biochemical reactions which the
soluble carbohydrates undergo afford such excellent illustrations of the
relation of the molecular configuration of an organic compound to the
possibility of the action of an enzyme upon it, that it seems desirable to
discuss this relationship at this point, rather than to postpone it until
after the nature of enzyme action has been considered. Undoubtedly, the
student, after he has studied the nature of enzymes and their mode of
action, as presented in Chapter XIV, will find it profitable to return to
this section and review the facts here presented, as illustrating the
principles and mechanism of enzyme action. But a consideration, at this
time, of the relation of the molecular configuration of the sugars to their
biochemical reactions cannot fail to add interest to the study of these
matters from the chemical and biological standpoints.

It has been known for a long time that the dextro- and levo-isomers of a
compound which contains one or more asymmetric carbon atoms are affected
differently by biological agents, such as yeasts, moulds, bacteria, etc.
Pasteur, as early as 1850, showed that the green mould, _Penicillium
glaucum_, when growing in solutions of racemic acid (a mixture of equal
molecules of _d_- and _l_-tartaric acids) uses up only the _d_-acid,
leaving the _l_-form absolutely untouched. Later, it was found that the
same green mould attacks _l_-mandelic acid in preference to the _d_- form;
whereas the yeast, _Saccharomyces ellipsoideus_, exhibits the opposite
preference for these acids.

These observations upon some of the earlier known forms of optically active
organic acids led the way to a general study of this phenomenon as
exhibited by the optically active soluble carbohydrates. The results of
these studies may be considered in connection with the several different
types of reactions which these sugars undergo, as follows:

=Glucoside Hydrolysis.=--As was pointed out in connection with the
discussion of the mutarotation of glucose, this sugar may exist in either
the [alpha] or the [beta] modification. Glucosides of both [alpha] and
[beta] glucose are of common occurrence. The difference in molecular
configuration, in such cases, may be represented by the following formulas:

R-O