Question

How many electrons, protons and photons are involved in the lysis of water to evolve one molecule of oxygen?

• A. $4e^{–},4H^{+}$ and 4 photons
• B. $4e^{–},4H^{+}$ and 8 photons
• C. $2e^{–},2H^{+}$ and 8 photons
• D. $2e^{–},2H^{+}$ and 4 photons

Answer 1

The correct option is B $4e^{–},4H^{+}$ and 8 photons

Four electrons must be moved through the photosynthetic electron transport pathway to convert two water molecules into one $O_2$ molecule (plus 4 hydrogen ions), and each electron requires two photons (one at each reaction centre) to traverse the pathway. Therefore, the quantum of light required in photosynthesis is 8 photons for one molecule of oxygen to be evolved. $2\times 2H_{2}O\to 4H^{+}+O_2+4e^{-}$