Question

Photolysis of each water molecule in light reaction will yield

• A. 2 electrons and 4 protons
• B. 4 electrons and 4 protons
• C. 4 electrons and 3 protons
• D. 2 electrons and 2 protons

Answer 1

Answer: (D)

Photolysis of water is also known as Hill Reaction. Robert Hill first proved this. He proved that if the chloroplasts extracted are suspended in a test tube containing suitable electron acceptors, like Potassium ferrioxalate or potassium ferricyanide, oxygen is released due to photochemical splitting of water. He extracted the chloroplast from the leaves of Stellaria media and Lamium album. Under these conditions, no CO₂ was consumed and no carbohydrate was produced, but the light-driven reduction of the electron acceptors was accompanied, by O₂ evolution.

$2H_{2}O{\to }_{OEC,C{a}^{2+},C{l}^{-}}^{Light}{O}_2+2H^{+}+2e^{-}$
Photolysis is catalyzed by the protein-bound inorganic complex containing manganese ions (oxygen-evolving complex) of photosystem II. This splitting of water to release molecular O₂ and H⁺ during photosynthesis is called photolysis.
Hill reaction proves that-
(i) In photosynthesis, oxygen is released from water.
(ii) Electrons for the reduction of CO₂ are obtained from water i.e., a reduced substance (hydrogen donor) is produced which later reduces CO₂.