Help understanding ROS-mediated responses

Oxygen is toxic to organisms only because of the highly reactive alternative forms of the molecule. These forms of oxygen can accumulate in cells and damage them by oxidizing key cellular molecules and structures. To tolerate the presence of oxygen (to allow aerobic cellular respiration to take place), cells must have ways of converting these toxic forms of oxygen to less harmful molecules. Cells use a variety of enzymes to accomplish this. These enzymes are superoxide dismutase (for elimination of the superoxide radical) and catalase and peroxidase (for the removal of hydrogen peroxide from cells). By continuous use of these enzymes, aerobic cells can use oxygen in their metabolism without suffering too many of the adverse effects of oxygen.