8 THE PRESIDENTIAL ADDRESS 
or enterprise, till only a few years ago they were shown to have 
great importance for industry. Yet catalysis is one of the most 
significant devices of nature, since it has endowed living systems 
with their fundamental character as transformers of energy, and 
all evidence suggests that it must have played an indispensable 
part in the living universe from the earliest stages of evolution. 
The catalysts of a living cell are the enzymic structures which 
display their influences at the surface of colloidal particles or at other 
surfaces within the cell. Current research continues to add to the 
great number of these enzymes which can be separated from, or 
recognised in, living cells and tissues, and to increase our knowledge 
of their individual functions. 
A molecule within the system of the cell may remain in an inactive 
state and enter into no reactions until at one such surface it comes 
in contact with an enzymic structure which displays certain adjust- 
ments to its own structure. While in such association the inactive 
molecule becomes (to use a current term) ‘ activated,’ and then enters 
on some definite path of change. The one aspect of enzymic 
catalysis which for the sake of my theme I wish to emphasise is its 
high specificity. An enzyme is in general adjusted to come into 
effective relations with one kind of molecule only, or at most with 
molecules closely related in their structure. Evidence based on 
kinetics justifies the belief that some sort of chemical combination 
between enzyme and related molecule precedes the activation of 
the latter, and for such combinations there must be close correlation 
in structure. Many will remember that long ago Emil Fischer 
recognised that enzymic action distinguishes even between two 
optical isomers and spoke of the necessary relation being as close 
as that of key and lock. 
There is an important consequence of this high specificity in 
biological catalysis to which I will direct your special attention. 
A living cell is the seat of a multitude of reactions, and in order 
that it should retain in a given environment its individual identity 
as an organism, these reactions must be highly organised. They 
must be of determined nature and proceed mutually adjusted with 
respect to velocity, sequence, and in all other relations. ‘They must 
be in dynamic equilibrium as a whole and must return to it after 
disturbance. Now if of any group of catalysts, such as are found 
in the equipment of a cell, each one exerts limited and highly- 
specific influence, this very specificity must be a potent factor in 
making for organisation. 
Consider the case of any individual cell in due relations with 
its environment, whether an internal environment as in the case of 
the tissue cells of higher animals, or an external environment as 
in the case of unicellular organisms. Materials for maintenance of 
