Question

In Z-scheme of electron transport, water splitting complex supplies electrons to
A. $$Photosystem-I$$
B. Plastoquinone
C. Pheophytin
D. $$Photosystem-II$$

Answer 1

Hint: During the non-cyclic flow of electrons, the two photosystems are connected in the series as $$PS-II$$ and $$PS-I$$. The Z-scheme or non-cyclic flow of electrons begins with the absorption of a photon by $$PS-II$$followed by$$PS-I$$I. When $$PS-II$$ absorbs the photon, it excites and transfer the electrons to Pheophytin which in turn passes the electron to $$PS-I$$.

Complete answer:
Step 1: The splitting of water is catalysed by the oxygen evolving complex (OEC), a protein complex present on the luminal surface of the thylakoid membrane. OEC oxidizes water and passes on the 4 electrons one at a time to the $$p680$$ ($$PS-II$$).
Step 2: After receiving the electron, $$p680$$ excites to $$p680\ast$$. This excited reaction centre then passes on the electrons to Pheophytin, a chlorophyll in which the Magnesium ion in the centre has been replaced by two hydrogen atoms. Pheophytin transfers the electrons to the $$PlastoquinoneQA$$and $$QB$$. $$Plastoquinone$$ pass the electrons to the $$Cytochromeb6f$$complex which in turn passes these electrons to the Plastocyanin, a blue-coloured Copper containing protein.
Step 3: $$PS-I$$ receives the electrons from Plastocyanin and after absorption of a photon get excited from $$p700$$ to $$p700\ast$$. The excited $$PS-I$$ then passes these electrons to the primary electron acceptor A0 which then passes the electrons to A1 which is a phylloquinone also called $$VitaminK1$$.
Step 4: A1 then pass the electrons to a series of Iron-sulfur proteins ($$F_X{F}_{A}and{F}_B$$) which finally transfers the electrons to soluble Ferredoxin followed by FNR (Flavoprotein- Ferredoxin-NADP reductase) which reduces the $$NAD{P}^{+}$$to $$NADPH$$.

Hence, the correct answer is option (D).

Note: The $$PS-I$$ was discovered by Louis Duysens and the $$PS-II$$ was discovered by Pierre Joliot. These two photosystems contain many pigments which help to trap light energy and convert them to chemical energy.