194
ERNEST SCHOFFENIELS
On the contrary, it appears that the limitation of enzymatic activity to
such a medium would be a secondary adaptation to a primitive condition.
This would also be the case for the mechanism generating an action
potential, since it utilizes the free energy accumulated as ionic gradients
to produce an electric current.
This general hypothesis may also explain why the active transport
of organic ions and molecules is possible only in the presence of adequate amounts of inorganic cations. Scheme I may therefore be proposed
to illustrate the relationships existing between the various functions of
a living membrane and the general cellular metabolism.
Synthesis of
higher molecular
.
weight units
Intracellular
Ionic gradient
Resting
potential
Action
potential
SCHEME I
It is obvious from this scheme that it is possible to modify one
function of the membrane without a direct action on the mechanism
directly responsible for this function. We may therefore expect a diversity in the sites of action of the various substances known to affect the
permeability characteristics, and this invites us to be cautious in interpreting the results obtained.
As a corollary it is evident that the identification of the molecular
architecture responsible for a permeability characteristic ("carrier,"
acetylcholine receptor, and so on) cannot be based solely on studies
of the binding properties of the material extracted for various substances
known to affect the function of the membrane in vivo.