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Q.
Identify $A, \, B, \, C, \, D$ and $E$ in the given flow chart showing Z-scheme of light reaction.
NTA AbhyasNTA Abhyas 2022
Solution:
Non-cyclic photo phosphorylation is the process, thereby the synthesis of ATP occurs due to the movement of the electrons in a non-cyclic manner using the energy from excited electrons provided by photosystem II. In non-cyclic photo phosphorylation, both PS I and PS II act in series and photolysis of water occurs. In this process, the formation of ATP and NADPH occurs, with the simultaneous release of oxygen. The non-cyclic photo phosphorylation is also called the Z scheme, due to its characteristic shape.
As the light energy is absorbed by pigments of PS II, these pigments transfer their energy to the PS II reaction centre-P680. The reaction centre-P680 becomes photo-excited and exudes an electron and becomes oxidized. The oxidized $P680^+$ must get an electron to return to the ground state, which comes from water by the process called photo-oxidation or photolysis of water. This process occurs in the oxygen-evolving complex located on the inner side or lumen side of the thylakoid membrane. Thus, the electrons released from water are immediately accepted by the oxidized reaction centre of PS II (i.e., $P680^+$.
The exudate electron of PS II is taken by pheophytin (quinone), which functions as the primary electron acceptor of PS II. The quinone sends them to an electron transport system consisting of plastoquinone, cytochrome complex and plastocyanin (a copper-containing protein). The electrons of plastocyanin are picked by PS I.
Simultaneously, the pigment molecules of PS I complex absorb solar radiation and transfer their energy to the PS I reaction centre- P700. Now, P700 gets excited and exudes an electron and itself becomes oxidized. The oxidized reaction centre of PS I ($P700^{+}$) takes an electron from plastocyanin.
The exudate electron of PS I is accepted by an iron-sulphur protein, the primary electron acceptor of PS I. It transfers its electron to ferredoxin (an iron-containing protein), that donates its electrons to $NADP^+$ which gets reduced to NADPH in the presence of enzyme Ferredoxin-NADP-reductase.
