Question

The energetic efficiency (in terms of number of ATP equivalents per absorbed photon) of non-cyclic electron transport process in photosynthesis, after taking into account the ATPs yielded by NADPH produced in it, is:

• A. 0.5
• B. 0.667
• C. 1.25
• D. 4.0

Hint:

While non-cyclic electron transport produces a fixed ratio of ATP to NADPH, plants can adjust this ratio using cyclic photophosphorylation, which generates ATP without producing additional NADPH to meet the exact energetic demands of the Calvin cycle.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Non-cyclic photophosphorylation (Z-scheme) is the light-driven process in photosynthesis where electrons flow from water to NADP${}^+$.
This process generates both ATP (via a proton motive force across the thylakoid membrane) and NADPH, storing energy for carbon fixation.
Key Formula or Approach:
To calculate the energetic efficiency in terms of ATP equivalents per absorbed photon, we use the stoichiometry of non-cyclic electron transport:
\[ \text{Efficiency} = \frac{\text{Direct ATP Produced} + \text{ATP Equivalent of NADPH}}{\text{Total Photons Absorbed}} \]

Step 2: Detailed Explanation:
Let us determine the stoichiometry of non-cyclic electron flow:
1. Photons absorbed: The transfer of 4 electrons from 2 $\text{H}_2\text{O}$ to produce 1 $\text{O}_2$ and 2 NADPH requires the absorption of 8 photons (4 by Photosystem II and 4 by Photosystem I).
2. Proton translocation: The flow of these 4 electrons leads to the translocation of 12 protons into the thylakoid lumen (8 protons via the Q-cycle and plastoquinol oxidation, plus 4 protons released directly from water splitting).
3. Direct ATP synthesis: The chloroplast ATP synthase requires approximately $4$ protons ($\text{H}^+$) to synthesize 1 ATP.
Thus, the 12 protons translocated can drive the synthesis of:
\[ 12 \text{ H}^+ / 4 \text{ H}^+ \text{ per ATP} = 3 \text{ ATP} \]
4. NADPH to ATP conversion: Each NADPH molecule has high reducing power, equivalent to approximately $3.5$ ATP molecules in terms of energetic value (or $3$ to $4$ ATP equivalents in metabolic calculations).
For 2 NADPH produced:
\[ 2 \text{ NADPH} \times 3.5 \text{ ATP equivalents} = 7 \text{ ATP} \]
5. Total ATP equivalents:
\[ \text{Total ATP equivalents} = 3 \text{ ATP (direct)} + 7 \text{ ATP (from NADPH)} = 10 \text{ ATP} \]
6. Efficiency:
\[ \text{Efficiency} = \frac{10 \text{ ATP equivalents}}{8 \text{ photons}} = 1.25 \text{ ATP per photon} \]

Step 3: Final Answer:
The energetic efficiency of the non-cyclic electron transport process is 1.25 ATP equivalents per absorbed photon, which corresponds to option (C).