In the absence of abundant oxygen, the RuBP carboxylase / oxidase - present in both C3 and C4 plants - fixes carbon dioxide efficiently, integrating it into the Calvin cycle. However, when the leaf's carbon dioxide concentration is low in relation to the oxygen concentration, the same enzyme catalyzes the reaction of the RuBP with the oxygen rather than with the carbon dioxide, acting as oxidase. This reaction leads to the formation of glycolic acid, the substrate for a process known as photorespiration.
Explanation:
Glycolic acid leaves the chloroplasts and enters the peroxisomes of photosynthetic cells. There, it oxidizes and, through certain chemical reactions, hydrogen peroxide and the amino acid glycine are formed. Two glycine molecules thus produced are transported to the mitochondria. There they are transformed into a molecule of serine (another amino acid), one of carbon dioxide and one of ammonia. This process, then, leads not only to fixation but to the loss of a molecule of carbon dioxide, therefore decreases the performance of photosynthesis. This process can occur during carbon fixation.
However, unlike mitochondrial respiration itself, photorespiration does not produce ATP or NADH. Under normal atmospheric conditions, up to 50% of the carbon fixed during photosynthesis can be reoxidized to CO2 during photorespiration. Thus, photorespiration greatly reduces the photosynthetic efficiency of some plants.
