Melvin Calvin (1911-1997) was awarded the 1961 Nobel Prize in Chemistry for his research on how plants assimilate carbon dioxide. Using the carbon-14 isotope as a tracer, Calvin and colleagues discovered the path followed by carbon through a plant during photosynthesis, starting from its absorption from atmospheric carbon dioxide to its conversion into carbohydrates and other organic compounds. Later, they showed that sunlight helps plants make organic compounds by acting on chlorophyll rather than the absorbed carbon dioxide, as previously believed. This process is called the Calvin cycle. Calvin also studied plant oils as energy sources, the chemical evolution of life, and radiation chemistry.