Question

In the light reactions of photosynthesis, the sequence of electron flow from the exited P680 (PS II) to P700 (PSI) during non-cyclic photophosphorylation is:

• A. $\text{P680}^* \rightarrow \text{Q}_\text{A} \rightarrow \text{Q}_\text{B} \rightarrow \text{Pheo} \rightarrow \text{Cyt } b_6f \text{ complex} \rightarrow \text{Fd} \rightarrow \text{P700}$
• B. $\text{P680}^* \rightarrow \text{Pheo} \rightarrow \text{Q}_\text{A} \rightarrow \text{Q}_\text{B} \rightarrow \text{Cyt } b_6f \text{ complex} \rightarrow \text{PC} \rightarrow \text{P700}$
• C. $\text{P680}^* \rightarrow \text{Q}_\text{A} \rightarrow \text{Q}_\text{B} \rightarrow \text{Pheo} \rightarrow \text{PC} \rightarrow \text{Cyt } b_6f \text{ complex} \rightarrow \text{Fd} \rightarrow \text{P700}$
• D. $\text{P680}^* \rightarrow \text{A}_0 \rightarrow \text{A}_1 \rightarrow \text{FeS}_\text{x} \rightarrow \text{Fd} \rightarrow \text{Cyt } b_6f \text{ complex} \rightarrow \text{PC} \rightarrow \text{P700}$

Hint:

To remember this sequence, use the abbreviation P-P-Q-Q-C-P-P: $\text{P680}^* \rightarrow$ Pheophytin $\rightarrow$ Q$_A \rightarrow$ Q$_B \rightarrow$ Cyt $b_6f \rightarrow$ Plastocyanin $\rightarrow$ P700.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Non-cyclic photophosphorylation (the Z-scheme) describes the flow of electrons through the photosynthetic electron transport chain in the thylakoid membrane.
Electrons are extracted from water, energized by light, and transferred from Photosystem II (PS II) to Photosystem I (PS I).

Step 2: Detailed Explanation:
The sequential steps of electron flow between the two photosystems are:
1. Excitation of PS II ($\text{P680}^*$): Absorption of light excites the reaction center chlorophyll of PS II to its high-energy state ($\text{P680}^*$).
2. Pheophytin ($\text{Pheo}$): The excited $\text{P680}^*$ transfers an electron to pheophytin, the primary electron acceptor of PS II.
3. Plastoquinones ($\text{Q}_\text{A}$ and $\text{Q}_\text{B}$): The electron moves from pheophytin to a tightly bound plastoquinone ($\text{Q}_\text{A}$), and then to a diffusible plastoquinone ($\text{Q}_\text{B}$).
4. Cytochrome $b_6f$ complex ($\text{Cyt } b_6f$): Fully reduced plastoquinone ($\text{PQH}_2$) transfers electrons to the cytochrome $b_6f$ complex, pumping protons into the thylakoid lumen.
5. Plastocyanin ($\text{PC}$): A copper-containing soluble protein transfers electrons from the cytochrome $b_6f$ complex to the oxidized reaction center of PS I ($\text{P700}$).
This sequence matches the path shown in Option 2.

Step 3: Final Answer:
The correct electron transport sequence is $\text{P680}^* \rightarrow \text{Pheo} \rightarrow \text{Q}_\text{A} \rightarrow \text{Q}_\text{B} \rightarrow \text{Cyt } b_6f \rightarrow \text{PC} \rightarrow \text{P700}$.