PSII Electron Transfer in Photosynthesis

The light reaction of photosynthesis involves the absorption of light energy by pigment molecules and the transfer of this energy to reaction centers, such as Photosystem II (PSII). In PSII, the reaction center chlorophyll, P680, absorbs light and becomes excited ($\text{P680}^*$). This excited electron is then transferred through a series of electron carriers.

Electron Carriers in PSII

Following the initial excitation of $\text{P680}$, the electron is transferred sequentially:

1. From $\text{P680}^*$ to a pheophytin molecule.
2. From pheophytin to $\text{Q}_\text{A}$ (plastoquinone A).
3. From $\text{Q}_\text{A}$ to $\text{Q}_\text{B}$ (plastoquinone B).

Both $\text{Q}_\text{A}$ and $\text{Q}_\text{B}$ are plastoquinone molecules that act as electron acceptors within PSII.

Analyzing the Statements

Let's examine each statement provided regarding the PSII electron carriers:

• A. The first electron released from reaction centre $\text{P680}$ is transferred to $\text{Q}_\text{A}$ to produce a plastosemiquinone.

  After excitation, $\text{P680}^*$ loses an electron, which eventually reaches $\text{Q}_\text{A}$. When $\text{Q}_\text{A}$ accepts this electron, it becomes reduced to a semiquinone form, $\text{Q}_\text{A}^-$. This is a plastosemiquinone. This statement accurately describes the initial electron transfer from $\text{P680}$ (via pheophytin) to $\text{Q}_\text{A}$ and the resulting state of $\text{Q}_\text{A}$.

• B. $\text{Q}_\text{A}$ is the mobile plastoquinone.

  $\text{Q}_\text{A}$ is the primary quinone acceptor in PSII. It is tightly bound to the PSII complex, specifically to the D2 subunit. It is not mobile; its role is to accept an electron from pheophytin and then transfer it to $\text{Q}_\text{B}$. The mobile plastoquinone is $\text{Q}_\text{B}$ after it has been fully reduced.

• C. The first electron transferred from $\text{Q}_\text{A}$ to $\text{Q}_\text{B}$ converts $\text{Q}_\text{B}$ into plastosemiquinone.

  $\text{Q}_\text{A}^-$ transfers an electron to $\text{Q}_\text{B}$. When $\text{Q}_\text{B}$ accepts this first electron, it becomes reduced to $\text{Q}_\text{B}^-$, which is a plastosemiquinone. However, $\text{Q}_\text{B}$ accepts a second electron from $\text{Q}_\text{A}^-$ (after another electron arrives from $\text{P680}$) and also takes up two protons from the stroma to become fully reduced plastoquinol ($\text{QH}_2$). While the first electron does form $\text{Q}_\text{B}^-$, the statement might be considered incomplete or misleading as $\text{Q}_\text{B}$'s functional role involves accepting two electrons and protons to become a mobile carrier, unlike $\text{Q}_\text{A}$ which remains bound as $\text{Q}_\text{A}^-$ until transferring the electron. Given that only A is stated as correct, C is likely considered incorrect because it focuses only on the intermediate step and doesn't fully reflect the process that leads to the mobile carrier.

• D. $\text{Q}_\text{B}$ is tightly bound to the complex and is not mobile.

  $\text{Q}_\text{B}$ is the secondary quinone acceptor. It is less tightly bound than $\text{Q}_\text{A}$. After accepting two electrons and two protons to become $\text{QH}_2$ (plastoquinol), $\text{Q}_\text{B}$ dissociates from PSII and moves into the lipid bilayer of the thylakoid membrane to transfer electrons to the cytochrome $\text{b}_6\text{f}$ complex. Thus, $\text{Q}_\text{B}$ is mobile after full reduction.

Conclusion

Based on the analysis of each statement:

• Statement A is correct.
• Statement B is incorrect because $\text{Q}_\text{A}$ is tightly bound and not mobile.
• Statement C describes an intermediate step, but $\text{Q}_\text{B}$'s main role involves accepting two electrons and becoming mobile, which the statement doesn't fully capture, leading to it being incorrect in this context.
• Statement D is incorrect because $\text{Q}_\text{B}$ is mobile after reduction.

Therefore, only statement A is correct.