Question

Identify the correct sequence of \(e^-\) flow leading to ATP synthesis in cyclic-photophosphorylation: [Question ID = 702][Question Description = 202_34_PLB_AUG22_Q102]

• A. P 700 \(\rightarrow\) PC \(\rightarrow\) Cyt \(b_6f\) complex \(\rightarrow\) \(A_0/A_1\) \(\rightarrow\) FeS \(\rightarrow\) Fd \(\rightarrow\) P700 [Option ID = 2805]
• B. P 700 \(\rightarrow\) Cyt \(b_6f\) complex \(\rightarrow\) \(A_0/A_1\) \(\rightarrow\) FeS \(\rightarrow\) Fd \(\rightarrow\) PC \(\rightarrow\) P700 [Option ID = 2806]
• C. P 700 \(\rightarrow\) Fd \(\rightarrow\) Cyt \(b_6f\) complex \(\rightarrow\) PC \(\rightarrow\) \(A_0/A_1\) \(\rightarrow\) FeS \(\rightarrow\) P700 [Option ID = 2807]
• D. P 700 \(\rightarrow\) \(A_0/A_1\) \(\rightarrow\) FeS \(\rightarrow\) Fd \(\rightarrow\) Cyt \(b_6f\) complex \(\rightarrow\) PC \(\rightarrow\) P700 [Option ID = 2808]

Hint:

Cyclic photophosphorylation occurs when the plant needs extra ATP relative to NADPH, or when \(CO_2\) levels are low, as it bypasses the reduction of \(NADP^+\).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Photophosphorylation is the process of generating ATP from ADP and inorganic phosphate using light energy. In cyclic photophosphorylation, only Photosystem I (PSI) is involved. Electrons expelled from the reaction center (P700) do not reduce \(NADP^+\) but instead cycle back to the reaction center through an electron transport chain, creating a proton gradient that drives ATP synthesis via the ATP synthase complex.

Step 2: Detailed Explanation:
The detailed sequence of electron carriers in PSI cyclic flow is as follows:
1. Excitation: Light energy is absorbed by the antenna complex and funneled to the reaction center chlorophyll \(a\), P700.
2. Primary Acceptors: The excited electron is transferred to the primary electron acceptors \(A_0\) (a chlorophyll \(a\) molecule) and then to \(A_1\) (phylloquinone).
3. Iron-Sulfur Centers: From \(A_1\), the electron moves through a series of membrane-bound iron-sulfur centers, collectively termed FeS (specifically \(F_X, F_B, F_A\)).
4. Ferredoxin (Fd): The electron then reaches the soluble protein Ferredoxin. In the cyclic mode, instead of moving toward FNR to reduce \(NADP^+\), Fd transfers the electron back to the Cytochrome \(b_6f\) complex.
5. Cyt \(b_6f\) and Plastocyanin (PC): The Cytochrome \(b_6f\) complex pumps protons across the thylakoid membrane. From here, the electron is passed to the mobile carrier Plastocyanin, which ultimately returns the electron to the oxidized P700, completing the cycle.
This flow corresponds to the sequence in Option D.

Step 3: Final Answer:
The correct sequence is P700 \(\rightarrow\) \(A_0/A_1\) \(\rightarrow\) FeS \(\rightarrow\) Fd \(\rightarrow\) Cyt \(b_6f\) complex \(\rightarrow\) PC \(\rightarrow\) P700.