Photosynthesis: Light Harvesting, Photosystems, Z-Scheme and Electron Transport - Plant Biotechnology | Agrigyan

During non-cyclic photosynthetic electron transport (the Z-scheme), absorption of a photon excites the reaction center chlorophyll of Photosystem II to $\text{P680}^*$. The precise sequential flow of electrons down the redox potential gradient to Photosystem I (P700) proceeds as follows:

Plastocyanin donates its electron to oxidize/neutralize the photo-oxidized $P700^+$.

• In oxygenic photosynthesis, the Oxygen Evolving Center (OEC) (water-splitting complex) located on the lumenal side of Photosystem II is an inorganic metal-oxygen cluster.
• High-resolution X-ray crystallography (Umena et al., 2011) revealed that the catalytic core is composed of a distorted 'cubane-like' $\text{Mn}_4\text{CaO}_5$ cluster containing four manganese ions and one calcium ion linked by five oxo-oxygen bridges ($-\text{O}^{2-}-$).
• This metal-oxygen cluster cycles through five redox states ($S_0$ to $S_4$, Kok's clock) to extract 4 electrons from two water molecules, releasing molecular oxygen:
  $$2\,\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\,\text{H}^+ + 4\,e^-$$
• Fe-S clusters are present in Photosystem I (e.g., $F_X, F_A, F_B$), not in the water-splitting complex of PSII.
• Thus, OEC is fundamentally a metal-oxygen cluster (Option 3).

• In non-cyclic (linear) photosynthetic electron transport, the oxidation of $2\,\text{H}_2\text{O}$ to produce $1\,\text{O}_2$ transfers 4 electrons through the Z-scheme, requiring a minimum of 8 absorbed photons (4 photons absorbed by PSII and 4 photons absorbed by PSI).
• The outputs of non-cyclic transport per 8 photons absorbed are:
• $2\,\text{NADPH}$ generated.
• Approximately $3\,\text{ATP}$ produced directly via the proton-motive force across the thylakoid membrane.
• In bioenergetic terms, each NADPH is biochemically equivalent to $3.5\,\text{ATP}$ equivalents (or 1 NADPH = 2.5–3.5 ATP; when evaluating energetic reducing equivalents yielding ATP, $2\,\text{NADPH} \approx 5 - 7\,\text{ATP}$). Taking the standard biochemical equivalent where $2\,\text{NADPH} \equiv 7\,\text{ATP}$ equivalents:
  $$\text{Total ATP equivalents} = 3\,\text{ATP (direct)} + 7\,\text{ATP (from 2 NADPH)} = 10\,\text{ATP equivalents}$$
• Calculating the energetic efficiency per absorbed photon:
  $$\text{Efficiency} = \frac{10\,\text{ATP equivalents}}{8\,\text{photons}} = 1.25\,\text{ATP equivalents per absorbed photon}$$
• Thus, Option 3 (1.25) is the correct energetic value.

• Statement I is correct: DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea], commercially known as Diuron, is a classical herbicide that acts as a potent inhibitor of photosynthetic electron transport during the light reactions of photosynthesis.
• Statement II is incorrect: DCMU specifically competes with plastoquinone for the $Q_B$ binding site on the D1 protein of Photosystem II (PS II) (not PS I). By blocking electron transfer from $Q_A^-$ to $Q_B$, it completely arrests the non-cyclic photosynthetic electron transport chain. (In contrast, bipyridylium herbicides like Paraquat accept electrons from PS I).
• Thus, Statement I is correct and Statement II is incorrect (Option 3).