3. CELLULAR ASPECTS OF ACTIVE TRANSPORT
155
transmitted by resonance under electronic pressure from the reactions of
the enzyme systems. If this is the primary source of electromotive force
in the membrane, it is then possible to explain the transport of ions,
assuming the existence of pores selective for different ions [see also
(91)].
3. Electrochemical Potential
Most workers, however, consider the potential difference as being
electrochemical in nature; the picture we arrive at is that, owing to
unequal distribution of ions and selective permeability characteristics
of the living membrane, a potential difference takes place according to
the relation shown in Eq. 3.
V
V
PTV.-FIn (
P k [ K i
]
+
P
"a[N a i ] + Pci[Ch}\
Ei-B,RT/zF In Vp K [K 2 ] + P Na [Na 2 ] + P 0 , [ c W
(
)
where E 1 — E 2 is the electrical potential difference existing between
solutions 1 and 2, the P's are the coefficients of relative permeability. The
subindexes 1 and 2 refer to solutions 1 and 2, respectively; R, T, z, and
F have their usual meaning (92). This relation is derived from the
constant field membrane of Goldman (93), i.e., the electric field developed across the membrane by the difference in ionic mobilities is continuous. This thus implies that the architecture responsible for the
permeability characteristics is homogeneous throughout. Equations have
also been derived considering that the electric field of the membrane
is distributed discontinuously across its thickness (94). Another limitation of Eq. 3 is that the P's are supposed to remain constant over a wide
range of concentration for the various ions considered. This is certainly
not the case, as will be shown below [Section VII; see also (92, 95)].
4. Significance of the Potential Difference
In the most generally accepted theory about the origin of bioelectric
potential, it is assumed that an electrical potential difference exists
owing to the selective permeability of the membrane and the unequal
distribution of ions. The following model of a cell may thus be proposed
(96) (Fig. 1). The intracellular concentration of Na, Na*
+
, is kept low
while that of K, Kj
+ is high, because of the active transport mechanism.
There is a leakage of Na and Κ through the structure responsible for the
passive permeability characteristics of the membrane, and the potential
difference is given by Eq. 4.
155
transmitted by resonance under electronic pressure from the reactions of
the enzyme systems. If this is the primary source of electromotive force
in the membrane, it is then possible to explain the transport of ions,
assuming the existence of pores selective for different ions [see also
(91)].
3. Electrochemical Potential
Most workers, however, consider the potential difference as being
electrochemical in nature; the picture we arrive at is that, owing to
unequal distribution of ions and selective permeability characteristics
of the living membrane, a potential difference takes place according to
the relation shown in Eq. 3.
V
V
PTV.-FIn (
P k [ K i
]
+
P
"a[N a i ] + Pci[Ch}\
Ei-B,RT/zF In Vp K [K 2 ] + P Na [Na 2 ] + P 0 , [ c W
(
)
where E 1 — E 2 is the electrical potential difference existing between
solutions 1 and 2, the P's are the coefficients of relative permeability. The
subindexes 1 and 2 refer to solutions 1 and 2, respectively; R, T, z, and
F have their usual meaning (92). This relation is derived from the
constant field membrane of Goldman (93), i.e., the electric field developed across the membrane by the difference in ionic mobilities is continuous. This thus implies that the architecture responsible for the
permeability characteristics is homogeneous throughout. Equations have
also been derived considering that the electric field of the membrane
is distributed discontinuously across its thickness (94). Another limitation of Eq. 3 is that the P's are supposed to remain constant over a wide
range of concentration for the various ions considered. This is certainly
not the case, as will be shown below [Section VII; see also (92, 95)].
4. Significance of the Potential Difference
In the most generally accepted theory about the origin of bioelectric
potential, it is assumed that an electrical potential difference exists
owing to the selective permeability of the membrane and the unequal
distribution of ions. The following model of a cell may thus be proposed
(96) (Fig. 1). The intracellular concentration of Na, Na*
+
, is kept low
while that of K, Kj
+ is high, because of the active transport mechanism.
There is a leakage of Na and Κ through the structure responsible for the
passive permeability characteristics of the membrane, and the potential
difference is given by Eq. 4.