Engineering Chemistry S Datta

PHOTOCHEMISTRY 557
–0.6
Bound
ferredoxin (P430)
Redox potential (V)
–0.4
–0.2
0
+0.2
Soluble
ferredoxin
NADPH
+0.4
P700
photosystem I
Photon
h
Fig. 25.6 Formation of NADPH by photosystem I.
Non-oxygen evolving photosynthetic bacterium contains PS-I only.
(b) Photosystem II. In oxygen-evolving systems PS-I and PS-II are serially linked to
produce NADPH and ATP by a two-step process.
–0.6
Bound (strong reductant)
ferredoxin
Redox potential (V)
0
Q (weak reductant)
e –
e – + +
NADP + H
NADPH
e –
+0.4
e – proton
gradient
ATP
Photosystem I
P700
(weak oxidant)
photon
h
+0.8 Photosystem II
photon h
P680
HO 2 O 2
+ 4H +
(strong oxidant: Mn-containing,
water-splitting enzyme)
Fig. 25.7 Electron transport and oxidation of H 2
O in noncyclic photo-phosphorylation (Z-scheme).
By absorbing light PS-II is raised to excited state and an electron is transferred to plasto
quinone (or) which in turn transfers it to cytochrome b 559. The final electron-carrier from