Question

Assertion (A): Splitting of water is associated with PS II. water splits into protons, O2 and electron.
Reason (R): The electrons needed to replace those removed from photosystem I are provided by PS II.

• A. Both (A) and (R) are correct and (R) is the correct explanation to (A)
• B. Both (A) and (R) are correct but (R) is not correct explanation for (A)
• C. (A) is correct (R) is wrong
• D. (A) is wrong (R) is correct

Hint:

Water splitting physically occurs on the inner side of the thylakoid membrane.
The protons generated accumulate in the thylakoid lumen, creating the proton gradient needed for ATP synthesis.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question presents an Assertion regarding the site and products of water splitting in photosynthesis, and a Reason concerning how electrons are supplied to Photosystem I (PS I).

Step 2: Key Concepts and Approach:
We evaluate the light reactions of photosynthesis (non-cyclic photophosphorylation / Z-scheme).
We analyze the role of Photosystem II (PS II) in water splitting and the subsequent path of electron transport to PS I.

Step 3: Detailed Explanation:

• Assertion (A): Photolysis (splitting) of water occurs at the oxygen-evolving complex associated with Photosystem II (PS II), which is located on the inner side of the thylakoid membrane.
The overall reaction is: \[ 2\text{H}_2\text{O} \rightarrow 4\text{H}^{+} + \text{O}_2 + 4e^{-} \] This reaction yields protons (\(\text{H}^{+}\)), molecular oxygen (\(\text{O}_2\)), and electrons. Thus, Assertion (A) is correct.

• Reason (R): When PS I is excited by light, its reaction center (P700) loses electrons.
These electrons are replaced by those passed down from PS II through an electron transport chain (Plastoquinone, Cytochrome \(b_6f\), and Plastocyanin). Thus, Reason (R) is correct.

• Connecting A and R: The primary physiological reason why water splits at PS II is to replace the electrons lost by the reaction center (P680) of PS II itself, not PS I directly.
The link between water splitting and PS I is indirect.
Therefore, while both statements are correct, Reason (R) is not the direct or correct explanation of Assertion (A).


Step 4: Final Answer:
Both statements are correct, but (R) is not the correct explanation of (A).