he enzyme, but the process cannot again be
repeated.

Calcium salts, or very dilute acids, are usually energetic activators of
proenzymes.

PHYSIOLOGICAL USES OF ENZYMES

There can be no doubt that enzymes exert a tremendously important influence
in vital phenomena, by determining the rate at which the chemical changes
which are involved in these phenomena shall proceed. Since they do not
initiate reactions, and since they may catalyze reversible reactions in
either direction, it cannot be said that they determine the type of
reactions which will take place in any given mass of protoplasm; but,
undoubtedly, they do exert a determining influence upon the rate at which
the reaction will proceed, after the protoplasmic activity has determined
the direction in which it shall go.

Without the intervention of these catalyzing agents, it would be impossible
for reactions between these non-ionized organic components of the cell
contents to come to completion with anything like the marvelous rapidity
with which these changes must take place in order to permit the organism to
grow, to perform its necessary vital functions, or to adjust itself to the
changes in its environmental conditions.

Since the number of different reactions which take place within a living
cell is very great, and since these chemical changes are extremely variable
in type, it follows that the number of different enzymes which must exist
in either a plant or an animal organism is likewise very large. For
example, fourteen different enzymes have been isolated from the digestive
system, and at least sixteen from the liver, of animals. They are
universally present in living protoplasm of every kind, from the most
minute bacterium to the largest forest trees, in the plant kingdom; and
from the am[oe]ba to the whale, in animals.

While there is a great variety of enzymes which may be produced by a single
individual organism, the same enzyme may be found in the greatest variety
of organisms; as, for example, the protease trypsin, which has been found
in several species of bacteria, in the carnivorous plant known as "Venus'
Fly Trap," and in the human pancreas, as well as that of all other animals.

FURTHER STUDIES NEEDED

From the discussions which have been presented in this chapter, it is
apparent that the enzymes play a tremendously important part in vital
phenomena, by controlling the rate at which t