Dioxygen in biological reactions

Free oxygen is produced in the biosphere through photolysis (light-driven oxidation and splitting) of water during photosynthesis in cyanobacteria, green algae, and plants.

During oxidative phosphorylation in cellular respiration, oxygen is reduced to water, thus closing the biological water-oxygen redox cycle.

[9] After being carried in blood to a body tissue in need of oxygen, O2 is handed off from the heme group to monooxygenase, an enzyme that also has an active site with an atom of iron.

Carbon dioxide, a waste product, is released from the cells and into the blood, where it is converted to bicarbonate or binds to hemoglobin for transport to the lungs.

Oxygen is used as an electron acceptor in mitochondria to generate chemical energy in the form of adenosine triphosphate (ATP) during oxidative phosphorylation.

[9] Parts of the immune system of higher organisms, however, create peroxide, superoxide, and singlet oxygen to destroy invading microbes.

[14] Reactive oxygen species also play an important role in the hypersensitive response of plants against pathogen attack.