\n– Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods with the help of chlorophyll pigment.
\n– The overall equation for photosynthesis is $6\\text{CO}_2 + 6\\text{H}_2\\text{O} + \\text{Light Energy} \\rightarrow \\text{C}_6\\text{H}_{12}\\text{O}_6 + 6\\text{O}_2$.
\n– The process occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
\n– The light-dependent reactions take place in the thylakoid membranes of chloroplasts. In this stage, light energy is absorbed by chlorophyll, leading to the splitting of water molecules ($H_2O$). This process is called photolysis.
\n– Photolysis of water ($2H_2O \\rightarrow 4e^- + 4H^+ + O_2$) releases electrons (used in the electron transport chain to produce ATP and NADPH), protons ($H^+$), and oxygen gas ($O_2$) as a byproduct. This oxygen is released into the atmosphere.
\n– The light-independent reactions (Calvin cycle) take place in the stroma of chloroplasts and use the ATP and NADPH produced during the light reactions to fix carbon dioxide ($CO_2$) from the atmosphere and synthesize carbohydrates.
\n– The question contains a slight inaccuracy by stating “$O_2$ of the atmosphere is fixed with the help of $H_2O$ and sunlight…”. It should be “$CO_2$ of the atmosphere is fixed…”. However, the second part of the sentence correctly asks what $O_2$ is evolved from.
\n– Based on the mechanism of photosynthesis, the oxygen gas evolved comes directly from the splitting of water molecules during the light-dependent stage.
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