Biomedical Engineering Reference
In-Depth Information
reentering the cell via the flagella motor apparatus, and thus powering the motion of the
flagellas. The first two possibilities would lead to wasteful dissipation of the converted
energy. They are prevented (or minimized) in the purple membrane by “step-function”
photoswitching (in the dark)
and
by having a relatively small area of phospholipid portion
in the two-dimensional crystalline structure of the purple membrane [13,15]. In the recon-
stituted bR BLM,
G
p
in the dark is considerably smaller than
G
m
. This result means that the
insulation against a proton backflow via the proton channel of bR
in the dark
is much more
effective than the insulation of phospholipid portion of the BLM. Quantitative comparison
of
G
p
with
G
m
in a native purple membrane is not available. But we think that the ratio
G
p
/
G
m
should be much greater in the native purple membrane than in our experimental
system because reconstitution in our experimental system could not have achieved the
same degree of high-density, crystalline packing of bR as in the native purple membrane.
From a mechanistic point of view, rectification in a photodiode is more important in the
dark than during illumination. While light-induced charge separation encounters the
forward resistance (1/
G
f
in Figure 15.14), charge recombination during illumination and
in the dark
encounters a much higher resistance (1/
G
r
in Figure 15.14). What transpires in
the purple membrane indicates that photon energy conversion is still possible without
rectification. The lack of rectification in the purple membrane may be the consequence of
the universal presence of reverse reactions in the scheme of coupled consecutive charge
transfer reactions (see later, Section 15.5). How
net
-forward charge transfers are still possi-
ble in spite of the lack of rectification is discussed in greater detail in Section 18 of [29].
Thus, by means of “step-function” photoswitching, bR achieves the same goal (in the dark,
at least) as does a silicon photodiode by means of rectification.
5
×
10
−
10
A
Valinomycin, 30 mM K
+
Control
100 ms
5 mV
10 s
On
On
On
Off
Off
Off
Off
Off
Light
Light
(A)
(B)
FIGURE 15.15
Photoelectric signals from photosynthetic membranes in response to a long square-wave light pulse. (A) The
open-circuit photovoltage was recorded intracellularly from an intact chloroplast of
Peperomia metallica
. The con-
trol signal, taken before the addition of valinomycin, is shown at left, and the effect of adding 1
M valinomycin
and 30 mM potassium ions in the external medium is shown at right. (B) The photosynthetic reaction center of
Rhodobacter sphaeroides
(8
M), supplemented with 100
M of ubiquinone-10, was reconstituted into BLM. One of
the two aqueous phases contained 25
M reduced cytochrome
c
, and the other contained 1 mM ferricyanide. See
text for discussion. (From Vredenberg, W. J., Bulychev, A. A. (1976). Changes in the electrical potential across the
thylakoid membranes of illuminated intact chloroplasts in the presence of membrane-modifying agents,
Plant
Sci. Lett.
7:101-107. (A) and Packham, N. K., Mueller, P., Dutton, P. L. (1988). Photoelectric currents across planar
bilayer membranes containing bacterial reaction centers: the response under conditions of multiple reaction-
center turnovers.
Biochim. Biophys. Acta
933:70-84. (B))
