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4.1 describe and discuss the importance of Van Neil’s hypothesis that water was the source of oxygen given out in photosynthesis
describe and discuss the importance of Van Niel’s hypothesis that water was the source of oxygen given out in photosynthesis
- Van Niel studied photosynthesis in bacteria that used H2S instead of H2O and produced sulfur solid in globules, rather than O2. These bacteria still required CO2 and produced carbohydrates (general formula CH2O). The equation for photosynthesis by the sulfur producing bacteria is
CO2 + 2H2S → CH2O + H2O + 2S
- Van Niel reasoned that the bacteria split the H2S (because the S was a product) and used the H to make sugar. He generalized that all photosynthetic organisms require hydrogen but the source from which they take varies, in green plants the H would come from splitting water. From this generalisation the equation for photosynthesis for green plants becomes
CO2 + 2H2O → CH2O + H2O + O2
The importance of van Niel’s hypothesis is that it gave one of the first clues to the mechanism of photosynthesis. Prior to Van Niel’s work, scientists had thought that carbon dioxide was split, the oxygen released and the C added to water to make glucose. Van Niel’s hypothesis that water is split to produce oxygen was later proved using radioactive oxygen – 18. Water labeled with 18O produced oxygen gas labeled with 18O. When CO2 labeled with 18O was used, the radioactivity was detected in the glucose and water, not the oxygen gas produced.