Calvin Wins the Nobel Prize for His Work on Photosynthesis

Date December 10, 1961

The 1961 Nobel Prize in Chemistry was awarded to Melvin Calvin for his studies of the molecular process of photosynthesis, named the Calvin cycle, which forms the basis for nutrition of all living things.

Locale Berkeley, California

Key Figures

• Melvin Calvin (1911-1997), American chemist and biochemist
• Michael Polanyi (1891-1976), English professor of chemistry and a fellow of the Royal Society with whom Calvin studied

Summary of Event

Unraveling the mystery of how green plants transform energy from the sun and carbon dioxide into sugars and oxygen has been a subject of interest and speculation for many years. It was not until Melvin Calvin published The Path of Carbon in Photosynthesis (1957) that the process was well understood. Calvin won the Nobel Prize in Chemistry in 1961 for his work.

Calvin’s interest in the fate of carbon in photosynthesis, a fundamental biochemical reaction, began while studying in England as a postdoctoral fellow with Michael Polanyi at the University of Manchester from 1935 to 1937. During these years, Calvin’s work involved coordinated metal compounds, molecules that have organic components attached to metals. There are many coordinated metal compounds that are significant biologically, such as heme, which is found in red blood cells, and chlorophyll, which is found in green plants. In 1937, Calvin left England and returned to the United States to teach at the University of California, Berkeley. From 1941 to 1945, Calvin’s work was directed at the war effort; this included two years on the Manhattan Project.

In 1946, Calvin was appointed director of the bioorganic division of the Lawrence Radiation Laboratory at Berkeley. His work centered on photosynthesis, the process that uses light energy, water, and carbon dioxide to produce oxygen and carbohydrates (sugars). Despite the pivotal role that photosynthesis plays in life, the process was not well understood, and the exact nature of the molecules involved in this series of reactions was unknown. Calvin approached the problem on two fronts: determining how light energy is utilized and how complex sugars and other molecules are formed. There had been speculation, even before 1940, that the conversion of carbon dioxide to carbohydrate might be a dark reaction, separate from the conversion of water to oxygen.

In 1937, Robert Hill had proven that oxygen could be produced from illuminated plant material when carbon dioxide was replaced. The reaction that occurs in the light, in addition to producing oxygen, formed high-energy reducing agents. These reducing agents, then, could convert carbon dioxide to sugar with no additional light. Calvin confirmed this observation by depriving plants of carbon dioxide while illuminating them so that they had the opportunity to store the high-energy reducing agents. When placed in a darkened environment, the plants were able to take up large amounts of carbon dioxide. Thus, incorporation of carbon dioxide depended on the reducing agents produced by light energy, rather than the light itself.

Calvin reasoned that if all chemicals involved in photosynthesis were identified, the sequence of reactions could be understood. Practical difficulties in such an undertaking were immense. Since all plant life is based on carbon, Calvin needed to determine how carbon derived from carbon dioxide could be distinguished from carbon from other sources. He postulated that if the carbon of carbon dioxide could somehow be “tagged,” the carbon might be followed in its path to carbohydrates or traced. The discovery of the long-lived radioactive isotope of carbon, carbon 14, by Samuel Ruben and Martin Kamen provided such a tool.

In 1945, an inexpensive and plentiful supply of carbon 14 was available with the construction of nuclear reactors. Carbon dioxide could be produced in which the carbon was radioactively tagged. Even when this carbon dioxide was transformed by the plant into other molecules, the source of the carbon would be announced to the researchers by its radioactivity. Naturally occurring carbon would not be detected in this way, so it could be ignored. Carbon dioxide entering plants appears eventually in all plant materials. In his Nobel address on December 11, Calvin described his strategy to investigate the chemicals that the carbon passes through by shortening “time of travel.” If the reactions that transform the carbon into various chemicals could be stopped before the carbon had progressed into too many intermediates, the identity of the molecules between carbon dioxide and sugars could be determined.

Many of the techniques that ultimately unlocked the secrets of the dark reactions of photosynthesis were developed as part of the war effort: radio isotopes, ion exchange, and paper chromatography. The apparatus used for Calvin’s studies was nicknamed “the lollipop” because of its shape. The alga chlorella was placed in the thin disklike apparatus and exposed to light and carbon dioxide. The tracer experiment was performed by substituting radioactive carbon dioxide in the stream. All chemical reactions were stopped by dropping the algae suspension into alcohol. This step also dissolved the organic molecules, separating them from the solid plant material, which could be analyzed for radioactivity. Traditional methods to purify such compounds were laborious and did not provide sufficient amounts of the materials to be analyzed. Calvin turned to ion exchange resins. When experimental mixtures were passed through columns of various resins and washed through with liquid, different compounds would pass out of the column at different speeds and thus could be separated.

It became obvious to Calvin that after only a few seconds the radioactive carbon had passed into a range of different compounds. The development of paper chromatography provided the means to separate the radioactive components of the photosynthetic process without knowing their identity. When different chemicals are placed on a porous sheet of paper and liquid is allowed to move up the paper, different molecules will move up the paper at varying speeds and are separated. If the paper is turned and liquid is allowed to move up at 90 degrees to its original direction, then the different compounds will be spread over the surface of the sheet. The location of molecules with radioactive carbon could be marked by allowing the paper to come into contact with photographic film. The areas of the film in contact with radioactive material would be exposed, creating dark patches. Some information to the identity of the molecules represented by dark patches was provided by subjecting the paper to analysis. More commonly, the separated materials were removed from the paper and subjected to chemical tests. The sequence of reactions suggested by these results proved to be complex.

By charting the amount of a particular molecule as it appeared and disappeared, Calvin could place that substance in a sequence. The photosynthetic reactions form a cycle, albeit not a logical one, at first glance. Three molecules of carbon dioxide combine with three molecules of ribulose biphosphate (RuBP, a five-carbon molecule). The result of the enzyme-catalyzed reaction is three molecules of a six-carbon molecule, which is bound to the enzyme. The molecules are cleaved into six three-carbon molecules, which have a phosphate group added to them by a highly energetic molecule adenosine triphosphate (ATP), produced in the light reaction. These products react with a reducing molecule from the light reactions of photosynthesis to produce six three-carbon molecules. One of these molecules is combined with an identical molecule from a second turn of the cycle. The remaining five molecules are transformed by a series of reactions to three molecules of RuBP, which can feed back into the cycle. At the time of the Nobel award, presented to Calvin on December 10, 1961, Calvin attributed the idea for the convoluted pathway to a trip he took with his wife. Inspiration came to him as he sat waiting in his car.

Significance

The insight into the photosynthetic processes provided by Calvin and colleagues at Berkeley presents a complex cyclic series of chemical reactions. This progression of reactions is referred to as the reductive pentose phosphate, or Calvin, cycle. Calvin was able to demonstrate the generality of the process he described as it occurred in a range of organisms from bacteria to higher plants. The light-dependent conversion of light energy to chemical energy as ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP) serves to transform reductively carbon dioxide to more complex organic molecules.

The molecules that result from photosynthesis—often referred to as photosynthates—form the basis for nutrition of all living things. Furthermore, they are the precursors to oil, fuel, petrochemicals, and all other materials derived from them, such as pharmaceuticals, plastics, feedstocks, and dyes. It is difficult to imagine a world without the products of photosynthesis, either directly or indirectly, since they touch almost every aspect of human life. Coupled with the knowledge Calvin provided into the molecular results of photosynthesis, an understanding of the energy utilization, or quantum conversion steps, has been strengthened. Calvin demonstrated that the structures involved with photosynthesis can be isolated and, moreover, the carbon dioxide reducing enzymes could be washed off, leaving only the equipment used for quantum conversion. Structural studies provided additional information about the chemical biodynamics of the photosynthetic transformation.

The work honored by the Nobel committee was characterized by its ambitious nature and its enormous attention to detail. Efraim Racker of the Public Health Research Institute of New York, an expert in photosynthesis also, said that “there was a lot of confusion in the field before Calvin. But he came up with the concept of the major cycle whereby carbon becomes sugar.” Noting there were many contributors in the field, Racker said, “Others helped, but Calvin instituted a broad attack on it. It was his imagination and brilliant conception that did it.” In addition to the enormous contributions to the biochemical field, Calvin creatively applied a wide range of analytical techniques to the project. Many of the techniques that proved pivotal to the work came out of the war effort.

Calvin proved to be a popular Nobel laureate. Time magazine referred to him as the “jolly biochemist” who had long been known as “Mr. Photosynthesis.” Calvin’s communication skills allowed his work to be widely understood and accepted.

Bibliography

Bassham, J. A., and M. Calvin. The Path of Carbon in Photosynthesis. Englewood Cliffs, N.J.: Prentice-Hall, 1957. A more technical discussion of the work, less accessible to general readers than the Nobel lecture (cited below). Includes detailed descriptions of the experiments performed and the rationale for the work. Contains a historical perspective of the photosynthesis problem. Bibliography.

Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer. Biochemistry. 6th ed. New York: W. H. Freeman, 2007. A massive, classic textbook for students of biochemistry. Includes chapters detailing “the light reactions of photosynthesis” and the Calvin cycle. Illustrations, maps, bibliography, index.

Calvin, Melvin. “The Long Journey.” In Science and Scientist, edited by M. Kageyama, K. Nakamura, T. Oshima, and T. Uchida. Tokyo: Japan Scientific Societies Press, 1981. Calvin discusses, for general readers, the generation of fuels and related materials by biological means. Calvin’s concern is to replenish the dwindling reserves of the products of “ancient photosynthesis.” The article describes the chemical history of fuel stocks and discusses the research done at Berkeley in Calvin’s laboratory on fuel-producing organisms.

‗‗‗‗‗‗‗. “The Path of Carbon in Photosynthesis.” Science 135 (March 16, 1962): 879-889. Includes personal insights and cites major influences. Contains pictures of the lollipop apparatus, the photographic film showing the blackened areas, detailed graphs illustrating some of the experiment, and diagrams indicating the cycle of reaction in detail.

Chemistry, 1942-1962. River Edge, N.J.: World Scientific, 1999. Part of the Nobel Lectures series, this collection includes the text of Calvin’s 1961 lecture.

Foyer, Christine H. Photosynthesis. New York: Wiley-Interscience, 1984. A textbook on photosynthesis with a broad base. The general concepts chapter will provide general readers with a good overview of the topic. Includes a clear, concise diagram to illustrate the text and many examples of experimental data. A technical bibliography is provided at the end of each chapter.

Lolich, Clarice. “Melvin Calvin.” In The Nobel Prize Winners: Chemistry, edited by Frank N. Magill. Vol. 2. Pasadena, Calif.: Salem Press, 1990. A brief but comprehensive discussion of Calvin’s work on photosynthesis, which led to his receiving the Nobel in 1961. Bibliography of primary and secondary works.