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The production of active oxygen species is inevitable in chloroplasts even under favorable environmental conditions. The photoreduction of oxygen has been observed even in washed thylakoids, depleted in ferredoxin. The overall stoichiometry of the thylakoid scavenging system shows no net change of oxygen and electron. Ground-state dioxygen occurs in the triplet state, and this is the reason why the reactivity of dioxygen with the singlet states of cellular components is very low, i.e., due to the difficulty arising from the addition of an electron to the antibonding orbital that is partially occupied. The photoreduction of oxygen has been observed even in washed thylakoids, depleted in ferredoxin. Therefore, membrane-bound reductants participate in the reduction of oxygen. Hydrogen peroxide is photooxidized, producing oxygen, in a reaction catalyzed by the photosystem II-water oxidase complex under flashed light …