Ribulose-1,5-bisphosphate carboxylase (rubisco) is the central enzyme of the Calvin Cycle because it catalyzes the initial reaction is stage I, in which CO2 is incorporated into ribulose-1,5-bisphosphate. This carboxylase activity occurs under conditions of low O2 and high CO2.

Under conditions of high O2 and low CO2, however, rubisco has oxygenase activity, initiating the phenomenon called photorespiration. When this happens, 3-phosphoglycerate and phosphoglycolate are formed in the chloroplast from ribulose-1,5-bisphosphate. The phosphoglycolate is then dephosphorylated and transferred into the peroxisomes (cell organelles) where glycolate is oxidized to H2O2 (destroyed by catalase) and glyoxylate (Figure 17.24). Glyoxylate is aminated to produce glycine, which is transferred to the mitochondria. There, two glycines are converted into one molecule of serine plus one molecule each of CO2 and NH3. The serine ultimately is converted back to 3-phosphoglycerate.

Photorespiration results in a net loss for the cell:

Ribulose-1,5-bisphosphate is lost from the Calvin cycle;

O2 is consumed, CO2 is released;

Only a part of the carbon is returned to the chloroplast;

ATP is expended without apparent benefit.

Plant cells probably undergo photorespiration for a reason, however. Under conditions of high illumination, CO2 levels around a plant may fall. Under these conditions, reactive oxygen species, such as superoxide, could be produced, causing damage to the plant. Photorespiration's function may be to protect against reactive oxygen species under these conditions by consuming oxygen and releasing CO2.