FIGURE 12-49 CO₂ fixation and photorespiration. These competing pathways are both initiated by ribulose 1,5-bisphosphate carboxylase (rubisco), and both use ribulose 1,5-bisphosphate. CO₂ fixation (pathway 1) is favored by high CO₂ and low O₂ concentrations; photorespiration (pathway 2) occurs at low CO₂ and high O₂ concentrations (that is, under normal atmospheric conditions). The phosphoglycolate produced by photorespiration is recycled via a complex set of reactions that take place in peroxisomes and in mitochondria, as well as in chloroplasts. The net result: for every two molecules of phosphoglycolate formed by photorespiration (four C atoms), one molecule of 3-phosphoglycerate is ultimately formed and recycled, and one molecule of CO₂ is lost.