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 \[p680*\]. 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reaction centre then passes on the electrons to Pheophytin, a chlorophyll in which the Magnesium ion in the centre has been replaced by two hydrogen atoms.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 \[Cytochrome b6 f\]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 \[p700*\]. 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 \[Vitamin K1\].
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.