Model Answer
0 min readIntroduction
Photosynthesis, the cornerstone of life on Earth, is the process by which plants and other organisms convert light energy into chemical energy. A crucial aspect of this process is the light-dependent reactions, initially studied by Robert Hill in 1939. The "Hill reaction," named after him, demonstrated that isolated chloroplasts could evolve oxygen in the presence of light and an artificial electron donor. This groundbreaking observation initially sparked debate about the origin of the evolved oxygen. However, subsequent experiments conclusively proved that the oxygen released during photosynthesis originates from water molecules, and not from carbon dioxide.
The Hill Reaction: A Brief Overview
The Hill reaction, also known as the "inverted photosynthesis," involves the isolation of chloroplasts from plant cells and exposing them to light in the presence of an artificial electron donor, such as ferricyanide (Fe(CN)₃). This bypasses the carbon dioxide fixation (dark reactions) and demonstrates that oxygen evolution is independent of CO₂. The reaction can be summarized as:
2H₂O + 2A → 2AH₂ + O₂
Where 'A' represents the artificial electron donor. This reaction highlights the role of light energy in splitting water and releasing oxygen.
Experimental Proof: Oxygen from Water, Not Carbon Dioxide
The initial assumption that CO₂ was the source of oxygen during photosynthesis was challenged by the landmark experiments of Samuel Ruben and Martin Kamen in 1941. They used heavy oxygen (¹⁸O) in two separate experiments:
Experiment 1: CO₂ as the Source
In the first experiment, plants were provided with ¹⁸O-labeled CO₂ (H₂¹⁸O). The resulting oxygen evolved was found to be devoid of ¹⁸O. This indicated that if CO₂ were the source of oxygen, the evolved oxygen would also be labeled with ¹⁸O. The absence of ¹⁸O in the evolved oxygen ruled out CO₂ as the source.
Experiment 2: H₂O as the Source
In the second experiment, plants were provided with ¹⁸O-labeled water (H₂¹⁸O). The oxygen evolved was found to be enriched with ¹⁸O. This confirmed that water was indeed the source of the evolved oxygen.
| Experiment | Labelled Substrate | Oxygen Evolved | Conclusion |
|---|---|---|---|
| 1 | ¹⁸O-labeled CO₂ | No ¹⁸O | CO₂ is not the source of oxygen |
| 2 | ¹⁸O-labeled H₂O | Enriched with ¹⁸O | H₂O is the source of oxygen |
The Overall Photosynthetic Equation
Based on these findings, the overall equation for photosynthesis is now understood to be:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This equation clearly demonstrates that oxygen is produced from the splitting of water molecules during the light-dependent reactions.
Mechanism of Oxygen Evolution
Oxygen evolution is catalyzed by the oxygen-evolving complex (OEC), a manganese-containing enzyme located within Photosystem II (PSII). The OEC facilitates the oxidation of two water molecules, releasing oxygen, protons (H+), and electrons. These electrons are then used to replenish those lost from Photosystem II.
Conclusion
In conclusion, the Hill reaction provided a crucial initial insight into the light-dependent reactions of photosynthesis. The subsequent experiments by Ruben and Kamen definitively established that the oxygen evolved during photosynthesis originates from water, not carbon dioxide. This understanding, underpinned by the oxygen-evolving complex within Photosystem II, is fundamental to our comprehension of the photosynthetic process and its vital role in sustaining life on Earth. Further research continues to refine our understanding of the intricacies of oxygen evolution within the photosynthetic apparatus.
Answer Length
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