The Path of Carbon in Photosynthesis: How to Discover a Biochemical Pathway

This paper traces the discovery of the so-called Calvin-Benson cycle of photosynthesis, that is, the cyclic sequence of reactions through which the reduction of carbon dioxide to carbohydrates is accomplished in the green organs of plants. This pathway, which herein is understood to be the model of a mechanism, was elucidated by a research team of the University of California at Berkeley in the years 1946-1954. The sequence of working hypotheses is described, with a focus on the methodology and heuristic strategies of the research group. While it has often been underlined that the group's innovative mixture of new techniques (radiotracers and paper chromatography) was the secret of its success, this paper describes how these techniques were applied: it transpires that the group combined the classic difference test, the interpretation of which was guided by additional heuristic assumptions and mechanistic thinking, with a specific kind of analogical inference, namely, the transfer of causal knowledge from one context to another.