Mapping the carbon reduction cycle: a personal retrospective.

The photosynthetic carbon reduction cycle was elucidated through the use of (14)CO(2) during photosynthesis to label metabolic intermediates. Mapping and proof of the cycle required identification of labeled metabolites, observation of changes in levels of labeled metabolites during transitions from light to dark and from high to low CO(2) levels, determination of intramolecular distribution of (14)C within the metabolites after a few seconds of photosynthesis with (14)CO(2), and estimation of metabolite concentrations, used to calculate true free energy changes at each step in the cycle.