The substrate that is oxidized during *photorespiration
plants. It is formed by the oxidation of ribulose bisphosphate to phosphoglyceric acid and phosphoglycolic acid. Hydrolysis of the phosphoglycolic acid yields free glycolic acid. During photorespiration the enzyme glycolate oxidase catalyses the oxidation, by molecular oxygen, of glycolic acid to glyoxylic acid. This reaction, which also produces hydrogen peroxide, takes place in the peroxisomes. The glyoxylic acid then appears to be transformed into glycine. In the cytoplasm two molecules of glycine react together to form serine and carbon dioxide. It is this carbon dioxide that is released in photorespiration.