The importance of the photosynthetic Gibbs effect in the elucidation of the Calvin-Benson-Bassham Cycle

Abstract The photosynthetic carbon reduction cycle, or Calvin-Benson-Bassham Cycle, is now contained in every standard biochemistry textbook. Although the cycle was already proposed in 1954, it is still subject of intense research, and even the structure of the cycle, i.e. the exact series of reactions, is still under debate. The controversy about the cycle’s structure was fuelled by the findings of Gibbs and Kandler in 1956 and 1957, when they observed that radioactive 14 CO 2 was dynamically incorporated in hexoses in a very atypical and asymmetrical way, a phenomenon later termed the ‘photosynthetic Gibbs effect’. Now, it is widely accepted that the photosynthetic Gibbs effect is not in contradiction to the reaction scheme proposed by Calvin, Benson and Bassham, but the arguments given have been largely qualitative and hand-waving. To fully appreciate the controversy and to understand the difficulties in interpreting the Gibbs effect, it is illustrative to illuminate the history of the discovery of the Calvin-Benson-Bassham Cycle. We here give an account of central scientific advances and discoveries, which were essential prerequisites for the elucidation of the cycle. Placing the historic discoveries in the context of the modern textbook pathway scheme illustrates the complexity of the cycle and demonstrates why especially dynamic labelling experiments are far from easy to interpret. We conclude by arguing that only mathematical models based on a sound theory are capable of resolving conflicting interpretations and providing a consistent quantitative explanation.