tween the machinery and its substrate. Fortunately, the dis- 

 covery of the long-lived isotopic carbons (carbon- 14) by 

 Samual Rubin and Martin Kamen in 1940 provided the 

 ideal tool for tracing these synthetic routes. 



In 1945, carbon- 14, radiocarbon, became available in 

 large amounts as a product from nuclear reactors. With the 

 encouragement and support of Professor Ernest O. Lawrence, 

 the Director of the Radiation Laboratory in Berkeley, Pro- 

 fessor Calvin began to study the pathway of carbon reduction 

 during photosynthesis, using carbon- 14 as his principal tool. 



Among a number of people who were to be associated 

 with him during the next few years of this work and who 

 would all contribute to the success of the research. Dr. Andrew 

 A. Benson was particularly instrumental, especially in the 

 identification of the early products of photosynthetic carbon 

 reduction. Key contributions to the development of the 

 carbon reduction cycle were made by Dr. Peter Massini and 

 Dr. Alex Wilson. Beginning as a graduate student with Pro- 

 fessor Calvin in 1947, I have had the pleasure of being asso- 

 ciated with him in this work to the present time. 



The first big success came with Professor Calvin's identi- 

 fication of phosphoglyceric acid as the first stable product of 

 carbon reduction during photosynthesis. Soon thereafter the 

 application of two-dimensional paper chromatography com- 

 bined with radioautography became an invaluable analytical 

 tool for separating the minute amounts of radioactive ingre- 

 dients formed in the plant. Identification of the remaining 

 intermediates in the carbon reduction cycle soon followed, 

 and these turned out to be all sugar phosphates. 



A combination of kinetic studies on the appearance of 

 carbon- 14 in these intermediates, with degradation of the 

 compounds that revealed the location of the radiocarbon in 

 individual atoms, soon led to a linking together of a reaction 

 sequence leading from phosphoglyceric acid through the 

 several sugar phosphates. The experiments of Massini and 

 Wilson helped to establish the carboxylation and reduction 

 reactions of photosynthesis, and the cycle was complete. 



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