Question

The correct order of light reaction elements is:
a. PSI
b. PSII
c. Plastocyanin
d. Plastoquinol
Choose the correct answer from the options given below:

• A. b, d, c, a
• B. b, c, d, a
• C. a, d, c, b
• D. b, c, a, d

Hint:

Remember that in the Z-scheme of photosynthesis, the electron flow starts at Photosystem II (PSII) and ends at Photosystem I (PSI).
Plastoquinol (PQ) serves as the lipid-soluble carrier right after PSII, whereas Plastocyanin (PC) serves as the water-soluble carrier right before PSI.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The light reactions of photosynthesis take place within the thylakoid membrane of the chloroplast.
The non-cyclic flow of electrons (described by the Z-scheme) involves the sequential transfer of electrons from water molecules to \( \text{NADP}^+ \), mediated by photosystems and mobile electron carriers.

Step 2: Detailed Explanation:
The sequential transport pathway of electrons through the light reaction components is as follows:
1. Photosystem II (PSII - b): The light reaction begins when light energy is absorbed by the reaction center chlorophyll (\( \text{P680} \)) of Photosystem II.
This absorption excites an electron, which is then transferred to pheophytin, leaving \( \text{P680} \) oxidized.
The oxidized \( \text{P680}^+ \) acts as a strong oxidant, extracting electrons from water molecules via the oxygen-evolving complex.
2. Plastoquinol (d): From pheophytin, electrons are transferred to plastoquinone (\( \text{PQ}_A \)) and then to \( \text{PQ}_B \).
Upon accepting two electrons and two protons from the stroma, plastoquinone is reduced to plastoquinol (\( \text{PQH}_2 \)).
Plastoquinol then diffuses through the lipid core of the thylakoid membrane to deliver these electrons to the Cytochrome \( b_6f \) complex.
3. Plastocyanin (c): The Cytochrome \( b_6f \) complex transfers the electrons to plastocyanin, a small, water-soluble, copper-containing peripheral membrane protein located on the luminal side of the thylakoid membrane.
Plastocyanin acts as a mobile carrier, transporting the electrons from the Cytochrome \( b_6f \) complex to the reaction center of Photosystem I.
4. Photosystem I (PSI - a): Plastocyanin reduces the photo-oxidized reaction center chlorophyll (\( \text{P700}^+ \)) of Photosystem I.
Following excitation of \( \text{P700} \) by light, the electrons are transferred through a series of iron-sulfur clusters to ferredoxin, and finally to \( \text{NADP}^+ \) by the enzyme ferredoxin-\( \text{NADP}^+ \) reductase.
Thus, the correct physical and sequential order of these elements is:
\[ \text{PSII (b)} \rightarrow \text{Plastoquinol (d)} \rightarrow \text{Plastocyanin (c)} \rightarrow \text{PSI (a)} \]

Step 3: Final Answer:
The correct chronological order of the elements is b, d, c, a, which corresponds to option (A).