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All of Melvin’s previous experiments guided him toward his eventual discovery of how photosynthesis is accomplished.  Melvin, through his many previous experiments, and using some experiments used by other chemists, was able to track the carbon molecule through the whole process, which he called “the path of carbon in photosynthesis.”  The radiocarbon isotope, 14C , was crucial in Melvin’s work, because the radioactive emissions made it possible to track where that carbon was in the cycle of reactions involving photosynthesis. 

It was known that the first step involved the conversion of carbon dioxide in the air into some compounds the plant could use for energy and growth. The initial question that Calvin asked was what were these compounds. By exposing plants to radioactive carbon dioxide and extracting various fractions from the plant, he could identify which compounds picked up the radioactive carbon.

Calvin first showed that the radioactive carbon of carbon dioxide got trapped in compounds that already existed inside the organelle, called a chloroplast because it has chlorophyll in it. Then he found that this fixed carbon was transferred into carbohydrates by a transfer of electrons that were energized by light (remember his earlier work on activated electrons with platinum-hydrogen bond?).  This notion came in handy here.  

He showed that photosynthesis actually consisted of two interacting series of actions: a light phase and a dark phase. The light phase involves the production of activated electrons and release of energy, both of which are used in the dark phase (now called the Calvin cycle) where carbon dioxide gets made into sugars. Calvin showed that the series of reactions involving trapping of carbon from carbon dioxide in air and moving it into sugars all occurred in the dark. Light was needed by the plant to generate energy and the activated electrons. The dark cycle consists of carbon moving through five different compounds that are regenerated. 

For this great achievement, Calvin was awarded the Nobel Prize in Chemistry in 1961. In Melvin’s acceptance speech at the Nobel Foundation in 1962, he said: “You have honored my colleagues, my family and me, but mostly my comrades in science.  I speak not only of those with whom I have had the pleasure to work directly – but the many others who preceded us and surround us in our work.  For each of us who appear to have had a successful experiment there are many to whom their own experiments seem barren and negative.  But they contribute their strength to the structure within which we all build.”

Between the Nobel Prize and his retirement, Melvin continued to do research and wrote a book about the chemical evolution of life, which was published in 1969.    In all, he had over 500 scientific papers and a total of seven books.  Even after retirement, Melvin continued to do research through the years.  His discoveries lead to the U.S. Department of Energy’s interest in the source of power from solar energy.  In addition to working with that department, Melvin also worked with the National Aeronautics and Space Administration (NASA) and the Executive office of the President.


Calvin, M. 1992. Following the Trail of Light. A Scientific Odyssey. American Chemical Society, Washington, D. C.

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