similar to photosystem II, while that of phototrophic green
bacteria is similar to photosystem I. Reaction centers are
included in the normal cytoplasmic membrane in green
bacteria or in invaginated cytoplasmic membrane in purple
bacteria. Because there is only one photosystem, the electron
transfer is cyclic (cyclic photophosphorylation) and allows
the production of chemical energy necessary for cellular
activities. Noncyclic transfer is necessary for the reduction
of coenzymes. This transfer requires energy and a reverse
flux of electrons for purple bacteria.
The redox potential of BChl a which is involved in the
reaction center is less electropositive than the O 2 /H 2 O redox
couple. Thus, the reduction of coenzymes requires electron
donors more reduced than water (reduced sulfur compounds,
+1
0,5
0
- 0,5
-1
-1,4
P680
Cyt b/f
P700
P700*
P680*
PC
A 0
A 1
Fe/S
Fd
NADP-reductase
Photon
Photon
NADP
+
NADPH, H
+
Ph
Q
PQ
H 2 O
1 2
/ O 2 + 2 H
+
E
‘
o ( Volts )
e
-
e
-
e
-
e
-
a
ATP
Photons
Photons
4 H
+
4 H
+
Fd
NADP
+
NADP
+
+
+
2 H
+
2 H
+
NADPH, H
+
ADP
PO4 33 H
+
3 H
+
ATPs
PC
1/2 O 2
2e
-
PQ
PS II
Cyt b/f
PS I
- red.
Thylakoid membrane
H 2 O
Inner thylakoid
Outside of thylakoid
b
Fig. 3.28 Schemes of oxygenic
photosynthesis. (a) The Z scheme
of electron flow. P680 and P700
chlorophyll of reaction centers of
photosystems II and I,
respectively, P680* and P700*:
excited chlorophylls, Ph
pheophytin, Q quinone, PQ
plastoquinone (substituted
quinones), Cyt b/f cytochromes b
and f complex, Pc plastocyanin
(copper protein), A0 and A1
electron acceptors (A 0,
chlorophyll amended), Fe/S
iron–sulfur protein, and Fd
ferredoxin (iron–sulfur protein).
(b) Location of the
photosynthetic apparatus in the
thylakoid membrane. PS I and PS
II photosystems I and II,
respectively, PQ plastoquinones,
PC plastocyanin, Fd ferredoxin,
NADP-red NADP reductase, and
ATPs ATP synthase. The
complex Cyt b/f plays the role of
proton pump (Drawing:
M.-J. Bodiou)
58
R. Matheron and P. Caumette
bacteria is similar to photosystem I. Reaction centers are
included in the normal cytoplasmic membrane in green
bacteria or in invaginated cytoplasmic membrane in purple
bacteria. Because there is only one photosystem, the electron
transfer is cyclic (cyclic photophosphorylation) and allows
the production of chemical energy necessary for cellular
activities. Noncyclic transfer is necessary for the reduction
of coenzymes. This transfer requires energy and a reverse
flux of electrons for purple bacteria.
The redox potential of BChl a which is involved in the
reaction center is less electropositive than the O 2 /H 2 O redox
couple. Thus, the reduction of coenzymes requires electron
donors more reduced than water (reduced sulfur compounds,
+1
0,5
0
- 0,5
-1
-1,4
P680
Cyt b/f
P700
P700*
P680*
PC
A 0
A 1
Fe/S
Fd
NADP-reductase
Photon
Photon
NADP
+
NADPH, H
+
Ph
Q
PQ
H 2 O
1 2
/ O 2 + 2 H
+
E
‘
o ( Volts )
e
-
e
-
e
-
e
-
a
ATP
Photons
Photons
4 H
+
4 H
+
Fd
NADP
+
NADP
+
+
+
2 H
+
2 H
+
NADPH, H
+
ADP
PO4 33 H
+
3 H
+
ATPs
PC
1/2 O 2
2e
-
PQ
PS II
Cyt b/f
PS I
- red.
Thylakoid membrane
H 2 O
Inner thylakoid
Outside of thylakoid
b
Fig. 3.28 Schemes of oxygenic
photosynthesis. (a) The Z scheme
of electron flow. P680 and P700
chlorophyll of reaction centers of
photosystems II and I,
respectively, P680* and P700*:
excited chlorophylls, Ph
pheophytin, Q quinone, PQ
plastoquinone (substituted
quinones), Cyt b/f cytochromes b
and f complex, Pc plastocyanin
(copper protein), A0 and A1
electron acceptors (A 0,
chlorophyll amended), Fe/S
iron–sulfur protein, and Fd
ferredoxin (iron–sulfur protein).
(b) Location of the
photosynthetic apparatus in the
thylakoid membrane. PS I and PS
II photosystems I and II,
respectively, PQ plastoquinones,
PC plastocyanin, Fd ferredoxin,
NADP-red NADP reductase, and
ATPs ATP synthase. The
complex Cyt b/f plays the role of
proton pump (Drawing:
M.-J. Bodiou)
58
R. Matheron and P. Caumette