THE PRESIDENTIAL ADDRESS 9 
the cell enter it from the environment. Discrimination among 
such materials is primarily determined by permeability relations, 
but of deeper significance in that selection is the specificity of the 
cell catalysts. It has often been said that the living cell differs 
from all non-living systems in its power of selecting from a hetero- 
geneous environment the right material for the maintenance of its 
structure and activities. It is, however, no vital act but the nature 
of its specific catalysts which determines what it effectively ‘ selects.’ 
If a molecule gains entry into the cell and meets no catalytic influence 
capable of activating it, nothing further happens save for certain 
ionic and osmotic adjustments. Any molecule which does meet 
an adjusted enzyme cannot fail to suffer change and become 
directed into some one of the paths of metabolism. It must here 
be remembered, moreover, that enzymes as specific catalysts not 
only promote reactions, but determine their direction. The glucose 
molecule, for example, though its inherent chemical potentialities 
are, of course, always the same, is converted into lactic acid by an 
enzyme system in muscle but into alcohol and carbon dioxide by 
another in the yeast cell. It is important to realise that diverse 
enzymes may act in succession and that specific catalysis has directive 
as well as selective powers. If it be syntheses in the cell which are 
most difficult to picture on such lines, we may remember that 
biological syntheses can be, and are, promoted by enzymes, and 
there are sufficient facts to justify the belief that a chain of specific 
enzymes can direct a complex synthesis along lines predetermined 
by the nature of the enzymes themselves. I should like to develop 
this aspect of the subject even further, but to do so might tax your 
patience. I should add that enzyme-control, though so important, 
is not the sole determinant of chemical organisation in a cell. Other 
aspects of its colloidal structure play their part. 
III. 
It is surely at that level of organisation, which is based on the 
exact co-ordination of a multitude of chemical events within it, 
that a living cell displays its peculiar sensitiveness to the influence 
of molecules of special nature when these enter it from without. 
The nature of very many organic molecules is such that they may 
enter a cell and exert noeffect. Those proper to metabolism follow, 
of course, the normal paths of change. Some few, on the other 
hand, influence the cell in very special ways. When such influence 
is highly specific in kind it means that some element of structure 
in the entrant molecule is adjusted to meet an aspect of molecular 
structure somewhere in the cell itself. We can easily understand 
that in a system so minute the intrusion even of a few such molecules 
B2 
