3. CELLULAR ASPECTS OF ACTIVE TRANSPORT
139
can be ruled out or disregarded, simple passive diffusion through any
membrane is characterized by Eq. 2 which is identical to Eq. 1 except
In M in /M ont
= In c 0 / Ci + zFE/RT
(2)
for the term expressing the solvent drag effect.
The application of Eq. 2 can be illustrated by the analysis of the
behavior of sodium ions in the isolated frog skin. Using Na
22 and Na
24 as
tracers, it is possible to determine influx and outflux simultaneously (2).
Table I shows some results obtained when the isolated skin is bathed
TABLE I
THE INFLUENCE OF 2,4-DINITEOPHENOL (DNP) ON INFLUX AND OUTFLUX
OF SODIUM THROUGH THE FROG SKIN°
Ε
Min/M 0 ut,
MiJMout,
Sample
M out
(mv)
found
calc.
Control
0.34
0.093
62
3.66
0.011
DNP
0.25
1.57
-11
0.16
0.15
Control
0.445
0.145
82
3.07
0.025
DNP
0.049
0.56
-6
0.087
0.126
Control
0.228
0.008
72
28.5
0.017
DNP
0.032
0.216
-7
0.148
0.132
a Skin is bathed with Ringer on the inside and Ringer diluted 1:10 on the outside (3).
with Ringer on the inside and Ringer diluted ten times on the outside
(3). In the periods marked control, the electrical potential difference
is rather high, the inside solution being positive with respect to the
outside. Nevertheless the influx of sodium is always greater than the
outflux, indicating a net movement of Na against the electrochemical
gradient. The values of the flux ratios found are very different from
those calculated according to Eq. 2. Thus the results indicate clearly
that physical forces cannot be responsible for the net flux of Na. Sodium
ions are therefore actively transported across the frog skin; that is,
metabolic energy has to be supplied to bring about the net flux observed.
In the presence of dinitrophenol (DNP), which is thought to dissociate
oxidation from adenosine triphosphate (ATP) synthesis, the behavior of
Na is that predicted from Eq. 2, indicating that under these conditions
sodium ions move according to the physical forces available.
During the last few years, evidence has been accumulated indicating
that in many instances where an unequal distribution of ions is observed
between two liquid phases separated by a living membrane or between
a cell and its surroundings, this is due to an active transport of one or
more of the ionic species involved. The literature on the subject has been
Min
139
can be ruled out or disregarded, simple passive diffusion through any
membrane is characterized by Eq. 2 which is identical to Eq. 1 except
In M in /M ont
= In c 0 / Ci + zFE/RT
(2)
for the term expressing the solvent drag effect.
The application of Eq. 2 can be illustrated by the analysis of the
behavior of sodium ions in the isolated frog skin. Using Na
22 and Na
24 as
tracers, it is possible to determine influx and outflux simultaneously (2).
Table I shows some results obtained when the isolated skin is bathed
TABLE I
THE INFLUENCE OF 2,4-DINITEOPHENOL (DNP) ON INFLUX AND OUTFLUX
OF SODIUM THROUGH THE FROG SKIN°
Ε
Min/M 0 ut,
MiJMout,
Sample
M out
(mv)
found
calc.
Control
0.34
0.093
62
3.66
0.011
DNP
0.25
1.57
-11
0.16
0.15
Control
0.445
0.145
82
3.07
0.025
DNP
0.049
0.56
-6
0.087
0.126
Control
0.228
0.008
72
28.5
0.017
DNP
0.032
0.216
-7
0.148
0.132
a Skin is bathed with Ringer on the inside and Ringer diluted 1:10 on the outside (3).
with Ringer on the inside and Ringer diluted ten times on the outside
(3). In the periods marked control, the electrical potential difference
is rather high, the inside solution being positive with respect to the
outside. Nevertheless the influx of sodium is always greater than the
outflux, indicating a net movement of Na against the electrochemical
gradient. The values of the flux ratios found are very different from
those calculated according to Eq. 2. Thus the results indicate clearly
that physical forces cannot be responsible for the net flux of Na. Sodium
ions are therefore actively transported across the frog skin; that is,
metabolic energy has to be supplied to bring about the net flux observed.
In the presence of dinitrophenol (DNP), which is thought to dissociate
oxidation from adenosine triphosphate (ATP) synthesis, the behavior of
Na is that predicted from Eq. 2, indicating that under these conditions
sodium ions move according to the physical forces available.
During the last few years, evidence has been accumulated indicating
that in many instances where an unequal distribution of ions is observed
between two liquid phases separated by a living membrane or between
a cell and its surroundings, this is due to an active transport of one or
more of the ionic species involved. The literature on the subject has been
Min