<Li> 2 H O → 2 H O + O (catalase) </Li> <Li> 2GSH + H O → GS--SG + 2H O (glutathione peroxidase) </Li> <P> Another type of reactive oxygen species is singlet oxygen (O) which is produced for example as byproduct of photosynthesis in plants . In the presence light and oxygen, photosensitizers such as chlorophyll may convert triplet (O) to singlet oxygen: </P> <Dl> <Dd> 3 O 2 → photosensitizer light 1 O 2 (\ displaystyle (\ ce ((^ (3) O2) -> ((\ ce (light))) ((\ ce (photosensitizer))) (^ (1) O2)))) </Dd> </Dl>

The chemical agent that produces highly toxic and reactive free radicals is