Perhaps no chemical process has a greater impact on life than the conversion of atmospheric carbon dioxide and water into carbohydrates by photosynthetic plants. Photosynthesis can be divided into two major reaction pathways. The first pathway, driven by light energy from the sun, uses electron transport and photophosphorylation to produce ATP and NADPH. These energy-rich compounds are then utilized in the second pathway to convert carbon dioxide into carbohydrate molecules—a process called carbon fixation. This latter pathway, called the Calvin cycle, was first elucidated by Melvin Calvin and colleagues at UC Berkeley.


Prior to 1940, investigating the chemical reactions involved in carbon fixation was hindered by the fact that it was difficult to distinguish the carbon atoms present in atmospheric CO2 from those contained in the substrates and products of the photosynthetic reactions. The subsequent development of radioactive tracers provided a tool for scientists to examine these pathways. By labeling carbon dioxide molecules using a heavy isotope of carbon, Calvin and colleagues were able to trace the integration of atmospheric carbon in the form of CO2 into a variety of compounds by photosynthetic organisms. By combining tracer labeling with paper chromatography, these researchers were able to establish ribulose 1,5-bisphosphate (RuBP) as the carbon dioxide acceptor molecule, as well as to elucidate the entire pathway of carbon fixation, sugar production, and regeneration of RuBP.

In recognition of the importance of these discoveries, Melvin Calvin was awarded the Nobel Prize in chemistry in 1961.

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Textbook Reference: Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates