Biomedical Engineering Reference
In-Depth Information
remaining off, the clamping voltage is returned to zero and the measured current is
brought back to level 1, again after a capacitative transient. This completes the cycle of
measurements in which two acts of measurement have been carried out: the measure-
ment of the photocurrent
I
p
(level 3 minus level 2), and the measurement of the photoemf
E
0
. The DC photoconductance,
G
p
, is then obtained by dividing the DC photocurrent with
the photoemf by virtue of Ohm's law.
I
E
p
(15.10)
G
p
0
By varying the value
V
c
in a systematic way, both the DC photocurrent and the photoemf
can be measured as a function of the transmembrane (clamping) voltage,
V
c
. The DC
photoconductance
G
p
can be determined as a function of the transmembrane voltage by
repeatedly using Eq. (15.10). Note that the background ionic conductance in the absence
of illumination,
G
m
, can also be obtained from the measured currents shown in Figure
15.12 by dividing the current (level 2 minus level 1) with the imposed voltage
V
c
, again by
virtue of Ohm's law.
We shall refer to the above-determined photoemf and the photoconductance as the
apparent photoemf
and the
apparent photoconductance
, respectively, since additional correc-
tions of these directly measured values are required when the restrictions imposed earlier
are relaxed. First, when
G
m
is comparable to
G
p
, the shunting effect of
G
m
must be taken
into account. Shunting will reduce the measured value of the apparent photoemf by a fac-
tor of
G
p
/(
G
p
G
m
), as is intuitively evident from the consideration of the effect of a volt-
age divider in elementary electronics. Again by intuitive reasoning, the apparent
photoconductance so measured actually contains a contribution from
G
m
, that is, the
apparent photoconductance represents the combined value of (
G
p
G
m
).
Another correction is required for the following reason. As measured experimentally,
G
p
turns out to be zero in the dark and is activated by illumination to a fixed nonzero value.
We shall refer to this feature as
“step-function” photoswitching
of the proton conductance
channel. In other words, the proton channel is either completely on or completely off, and
does not take on any intermediate values. In this case, when
G
p
suddenly turns nonzero
upon illumination, both
V
c
and
E
p
can each drive a current through
G
p
. Both currents will
be treated as photocurrents by the definition given above. Thus, the photocurrent must be
decomposed into two fractions: one driven by light and the other by the transmembrane
potential. Likewise, the apparent photoemf can be decomposed into two parts: the applied
voltage and the true photoemf. This correction would not be necessary if
G
p
had the same
value during illumination as in the dark because the
V
c
-driven current has already been
included as part of the dark current (e.g., see the DC photoelectric effect of the Mg por-
phyrin membrane [35,68,30]).
Thus, to correct the above-mentioned effects, Eqs. (9) and (10) must be replaced by
EE
GG
G
p
m
V
(15.11)
p
0
c
p
and
I
E
p
(15.12)
G
G
p
m
0
