Any of various chemical species that contain highly active oxygen, particularly the free radical superoxide anion (O₂⁻) and its derivatives, including hydrogen peroxide (H₂O₂), singlet oxygen (a metastable high-energy form of molecular oxygen), hydroxyl radical (OH·), and hypohalite ions (e.g. hypochlorite, OCl⁻). Superoxide is produced normally as a by-product of aerobic respiration inside mitochondria; both it and other ROS are potentially harmful to living cells because of their highly reactive nature. They can damage DNA and other cell components and are implicated in various diseases (e.g. cancer, heart disease), hence cells have an array of mechanisms to remove them, including superoxide dismutase and various antioxidants. However, certain cells and tissues in both plants and animals use a process called the oxidative burst to produce ROS in order to destroy invading pathogenic organisms, such as fungi and bacteria. See also peroxisome.