Melvin Calvin (1911-1997), American biochemist, in his photosynthesis research laboratory. Photosynthesis is the process by which green plants convert light, water and carbon dioxide into food and oxygen. Calvin began to study photosynthesis in 1946 using single-celled Chlorella algae. He identified a cycle of reactions by exposing the algae to traceable radioactive forms of carbon dioxide. His discovery of this universal photosynthetic process, called the reductive pentose phosphate or Calvin cycle, earned Calvin the 1961 Nobel Prize for Chemistry. Photographed in 1957.