Question

Quantum yield of oxygen production in the light reactions of photosynthesis is

• A. 10
• B. 1
• C. 0.1
• D. 0.01

Hint:

Remember:
- Quantum Requirement = $8-10$ photons per $\text{O}_2$ evolved.
- Quantum Yield = $0.1$ (or $10-12\%$) $\text{O}_2$ evolved per photon absorbed.
The value of quantum yield remains constant across most $\text{C}_3$ species under optimal conditions.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Quantum yield ($\Phi$) is defined as the number of photochemical products (such as oxygen molecules evolved or carbon dioxide molecules fixed) per absorbed photon of light.
It measures the efficiency of light energy conversion during the light-dependent reactions of photosynthesis.
Key Formula or Approach:
The relationship between quantum requirement (number of photons needed to produce one $\text{O}_2$ molecule) and quantum yield is reciprocal:
\[ \text{Quantum Yield } (\Phi) = \frac{1}{\text{Quantum Requirement}} \]

Step 2: Detailed Explanation:
During non-cyclic photophosphorylation, split-water reactions at Photosystem II (PS II) release one molecule of oxygen ($\text{O}_2$) from two molecules of water:
\[ 2H_2O \rightarrow \text{O}_2 + 4H^+ + 4e^- \]
Moving four electrons through the Z-scheme from water to $NADP^+$ requires a minimum of eight photons (four absorbed by PS II and four by PS I).
Under actual physiological conditions, energy losses mean the measured quantum requirement for releasing one molecule of $\text{O}_2$ is approximately 8 to 10 photons.
Using this minimum of 8 to 10 photons in our formula gives:
\[ \Phi = \frac{1}{8} \text{ to } \frac{1}{10} = 0.125 \text{ to } 0.10 \]
Therefore, the quantum yield of oxygen production in healthy leaves is approximately $0.1$ (or $10\%$).

Step 3: Final Answer:
The quantum yield of oxygen production is approximately 0.1.
This corresponds to Option (C).