3. CELLULAR ASPECTS OF ACTIVE TRANSPORT
185
178). The specific transverse resistance increases if the current is flowing
inward; the reverse is true if the current flows in the opposite direction.
The variations in specific resistance observed when the current is flowing are different for a same current density according to the direction of
flow. Consequently the electrical potential difference is not a linear
function of the current density and Ohm's law is not verified for many
living membranes. The resistance being different according to the direction of current flow, it is said that the living membrane has rectifying
properties. These electrical measurements thus show that the passive
permeabilities of the membrane to Na and to Κ are different and that
these permeabilities are differently affected by an electrical current.
Ca ions increase whereas Mg ions decrease the rectification properties
of a membrane (179). In Ca-free medium rectification of the squid giant
axon membrane disappears (178) and the specific transverse resistance
is approximatively proportional to the Ca concentration in the outside
medium (180). This change in resistance has to be related to the increase
in Na influx and Κ efflux, observed in Ca-free medium (134). The above
results clearly show that the molecular architectures responsible for the
permeability characteristics of a living membrane—to Na and Κ especially—are directly influenced by the concentration of ionized Ca in
the surrounding medium.
2. The Resting Potential
To assimilate a living cell and its surrounding medium to a simple
physicochemical system is a rather naive assumption. It is evident that
the chemical reactions of which the cell is the site, obey the laws of
physicochemistry and more specifically the second law of thermodynamics. However, the physicochemical science has not yet defined laws
enabling us to analyze and to define a biological function, considered
at the molecular level, by integration of the chemical reactions in a
sequence taking into account both physiological and morphological data.
The study of the effect of the ionic composition on cellular structures
has to be inserted in this task since such a study has for ultimate purpose
a precise definition of these structures. As far as the cellular membrane
is concerned we know that it is not the inert structure once postulated
in earlier theories on permeability. It is the site of active transport
mechanisms and is responsible for the generation of bioelectric potentials.
If the theory concerning the origin of electrochemical potential is of
great help in our understanding of bioelectric potentials, experimental
data show that this theory has to be applied to a biological system with
great caution. In the Nernst formulation, the membrane structure is not
supposed to be altered by a modification of the ionic composition of the
185
178). The specific transverse resistance increases if the current is flowing
inward; the reverse is true if the current flows in the opposite direction.
The variations in specific resistance observed when the current is flowing are different for a same current density according to the direction of
flow. Consequently the electrical potential difference is not a linear
function of the current density and Ohm's law is not verified for many
living membranes. The resistance being different according to the direction of current flow, it is said that the living membrane has rectifying
properties. These electrical measurements thus show that the passive
permeabilities of the membrane to Na and to Κ are different and that
these permeabilities are differently affected by an electrical current.
Ca ions increase whereas Mg ions decrease the rectification properties
of a membrane (179). In Ca-free medium rectification of the squid giant
axon membrane disappears (178) and the specific transverse resistance
is approximatively proportional to the Ca concentration in the outside
medium (180). This change in resistance has to be related to the increase
in Na influx and Κ efflux, observed in Ca-free medium (134). The above
results clearly show that the molecular architectures responsible for the
permeability characteristics of a living membrane—to Na and Κ especially—are directly influenced by the concentration of ionized Ca in
the surrounding medium.
2. The Resting Potential
To assimilate a living cell and its surrounding medium to a simple
physicochemical system is a rather naive assumption. It is evident that
the chemical reactions of which the cell is the site, obey the laws of
physicochemistry and more specifically the second law of thermodynamics. However, the physicochemical science has not yet defined laws
enabling us to analyze and to define a biological function, considered
at the molecular level, by integration of the chemical reactions in a
sequence taking into account both physiological and morphological data.
The study of the effect of the ionic composition on cellular structures
has to be inserted in this task since such a study has for ultimate purpose
a precise definition of these structures. As far as the cellular membrane
is concerned we know that it is not the inert structure once postulated
in earlier theories on permeability. It is the site of active transport
mechanisms and is responsible for the generation of bioelectric potentials.
If the theory concerning the origin of electrochemical potential is of
great help in our understanding of bioelectric potentials, experimental
data show that this theory has to be applied to a biological system with
great caution. In the Nernst formulation, the membrane structure is not
supposed to be altered by a modification of the ionic composition of the