The correct option is Water.
Explanation
Photosynthesis is an anabolic process where green plants synthesize organic food (carbohydrates) from carbon dioxide and water in the presence of sunlight and chlorophyll. The overall chemical equation is represented as:
6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2
Historically, it was believed that oxygen was released from the splitting of carbon dioxide. However, experimental evidence provided by Cornelius van Niel and later confirmed by Ruben and Kamen using radioisotopes (18O) proved otherwise.
Detailed Analysis
- Source of Oxygen (Water): The oxygen released into the atmosphere is a byproduct of the Light Reaction (photochemical phase). During this phase, light energy is utilized to split water molecules. This process is called the photolysis of water. The splitting of water releases electrons, protons (H+), and molecular oxygen (O2).
- Role of Carbon Dioxide: Carbon dioxide (CO2) is reduced during the Dark Reaction (Calvin Cycle) to form carbohydrates (glucose). The oxygen atoms present in the glucose molecule are derived from CO2, not the oxygen released as gas.
- Role of Chlorophyll: Chlorophyll acts as the primary pigment for trapping light energy required to drive the photolysis of water and subsequent electron transport, but it does not chemically decompose to release oxygen.
Key Takeaway: The oxygen evolved during photosynthesis comes exclusively from the photolysis of water occurring in the thylakoid lumen, not from carbon dioxide.