Oxygen molecule in the ground state is relatively reactive . However, it can be the source of the reactive derivative , called reactive oxygen species (ROS ) , both produced as a result of the metabolic activity of cells , as well as plants after exposure to various stress factors . In a progressive one-electron reduction of oxygen occurs to produce partially reduced intermediates , which are in the order of superoxide anion radical ( O2 - • ) , hydrogen peroxide (H2O2 ) and hydroxyl radical ( • OH) .