• Quantum requirement (QR) is defined as the number of absorbed photons (quanta) required to release one molecule of oxygen ($\text{O}_2$) during the light reactions of photosynthesis.
• The theoretical and experimentally verified minimum quantum requirement is approximately 8 to 10 photons per molecule of $\text{O}_2$ evolved (4 electrons transferred through the Z-scheme, requiring 2 photons per electron across PS II and PS I).
• Quantum yield ($\Phi$) is the mathematical reciprocal of the quantum requirement:
  $$\Phi = \frac{1}{\text{Quantum Requirement}} = \frac{1}{8 \text{ to } 10} = 0.125 \text{ to } 0.10$$
• Therefore, the quantum yield of oxygen evolution is approximately 0.1 (Option 3).

Linear electron flow pathway during chloroplast light reactions (Z-scheme):

• Photolysis of water oxidizes at Photosystem II (PSII / P680) -> (b).
• Reduced plastoquinone carries electrons to form Plastoquinol ($\text{PQH}_2$) -> (d).
• Plastoquinol transfers electrons via the Cytochrome $b_6f$ complex to the mobile copper protein Plastocyanin -> (c).
• Plastocyanin reduces photo-oxidized Photosystem I (PSI / P700) -> (a).

Role of Plastocyanin in thylakoid electron transport:

• Plastocyanin is a water-soluble, peripheral copper-binding metalloprotein located in the thylakoid lumen.
• Its single catalytic copper ion alternates between $\text{Cu}^{2+}$ (oxidized, blue) and $\text{Cu}^+$ (reduced, colorless) as it shuttles electrons from the Cytochrome $b_6f$ complex to the reaction center of Photosystem I (P700).
• Cytochrome c oxidase is a mitochondrial enzyme, plastoquinone is a lipophilic quinone, and riboflavin is a vitamin cofactor.

Historical experiments in photosynthesis:

• In 1770-1774, Joseph Priestley revealed that a mint plant in a bell jar restored the air that had been fouled by a burning candle or a suffocating mouse, leading to the discovery of oxygen evolution.
• Jan Ingenhousz (1779) showed that sunlight is essential for this purification and that green parts alone release oxygen.
• B. van Niel (1931) demonstrated using purple sulfur bacteria that oxygen comes from water ($\text{H}_2\text{O}$), not $\text{CO}_2$.
• T.W. Engelmann (1882) plotted the first action spectrum of photosynthesis using Cladophora and aerobic bacteria.