Engineering Chemistry S Datta

558 ENGINEERING CHEMISTRY
PS-II to PS-I is a plastocyanin (Cu containing protein). PS-I accepts electrons from PS-II. By
means of two-step electron-energising processes, catalysed by each photosystem, electrons are
finally transferred from H 2
O to the final acceptor bound-ferredoxin P430 (Fig. 25.7). Light
energy causes electrons to flow from H 2
O to NADPH.
The processes of photosynthetic system of P680 and P700 are summarized in Fig. 25.8.
Stroma side
+
H =+ve
Photon, h 2H +
Photon, h
ADP+Pi ATP
+ +
NADP + H NADP
PQ
Thylakoid
membrane
PSII
P680
Plastoquinone
loop
Cyt b -f
6
PSI
P700
ATPase
2e –
PQH 2
2e –
2e –
Lumen side
+
H =–ve
PC
HO 2 0.5O + 2H
2
2H + nH
+
electrochemical
proton gradient
PO = Plastoquinone; PC = Plastocyanin;
electron transport by electron carriers PQ and PC
Fig. 25.8 Schematic representation of energy transduction by reaction centres PS-II and PS-I in the
noncyclic photosynthetic reaction process (According to Tanz and Zieger, “Plant Physiology” Benjamin/
Cummings, Redwood City, CA. (1991).
The electron transport process creating electro-chemical proton gradient across the
thylakoid membrane drives the synthesis of ATP from ADP and P i
by ATP synthase.
4 hν
2H 2
O + 2NADP + + 3ADP 3– + 3HPO 2– ⎯⎯→ 2H + + O 2
+ 2NADPH + 3ATP 4–
Chloroplasts can also synthesize only ATP without producing NADPH involving cyclic
photophosphorylation.
Dark reactions. The NADPH and ATP formed by the ‘light reactions’ are utilized for
synthesizing carbohydrates from CO 2
in stroma for which sunlight is not required. These CO 2
fixing reactions are called Hill reactions, which can take place in dark.
Plants generate glyceraldehyde-3-phosphate (GAP) as the starting material for higher
carbohydrates from three molecules of CO 2
by utilising 9 ATP and 6 NADPH. This metabolic
pathway for carbohydrate synthesis is called Calvin-Benson cycle.
3CO 2
+ 9ATP + 6NADPH + H 2
O ⎯⎯→ GAP + 6NADP + + 8 P i
+ 9 ADP.