How are reactive oxygen species generated in living cells?

The majority of ROS are generated naturally as byproducts during mitochondrial electron transport. During oxidative phosphorylation, electrons leaking from the ETC react with molecular oxygen, causing the formation of superoxide radicals. ROS are also produced from NADPH oxidase as part of their normal function. Electrons are transferred from NADPH to molecular oxygen to generate superoxide anions. Neutrophils and macrophages produce ROS as a defense mechanism to destroy pathogens, and is part of the immune response. The enzyme myeloperoxidase generates HOCL by reacting hydrogen peroxide with chloride ions. HOCL plays an important role in innate immune processes. Environmental factors such as ionizing radiation and pollutants can also cause ROS generation in cells. They may directly interact with cellular components and induce oxidative stress. Additionally, ROS are produced as required intermediates in metal-catalyzed oxidation reactions.