English: Electrons donated from NADH and FADH2 pass through the electron transport chain and ultimately reduce oxygen to form water at complex IV. Mitochondrial ROS are produced from the leakage of electrons to form superoxide at complex I and complex III. Superoxide is produced within matrix at complex I, whereas at complex III superoxide is released towards both the matrix and the intermembrane space. Once generated, superoxide is dismutated to hydrogen peroxide by superoxide dismutase 1 (SOD1) in the intermembrane space and by SOD2 in the matrix. Afterwards, hydrogen peroxide is fully reduced to water by glutathione peroxidase (GPX). Both superoxide and hydrogen peroxide produced in this process are considered as mitochondrial ROS. OM: outer membrane; IM: inner membrane.