Chemically reactive molecules and ions of oxygen with an unpaired electron
Reactive oxygen species are molecules and ions containing reactive oxygen. By reactive, it means that the oxygen has an unpaired electron. In biological systems, the presence of reactive oxygen species affects metabolic activities of the cell. They have an effect in homoeostasis and cell signaling, and associated with certain manifestation of diseases. An increase of reactive oxygen species, for instance, could be damaging to the cell structures and biomolecules (e.g. proteins, lipids, and DNA). Some of the factors that increase their level are heat, metal toxicity, nutrient deficiency, UV radiation, ionizing radiation, etc. In cell signaling, they may act as cellular messengers and cause disruptions in the system. They are produced as a stress response of the cell to these factors. An excessive amounts of reactive oxygen species could lead to oxidative stress, which is defined as a disturbance caused by the accumulation of reactive oxygen species A highly oxidized environment inside the cell manifests when the biological system is unable to detoxify reactive intermediates. In humans, it is presumed to be involved in the development of cancers, Alzheimer’s disease, vitiligo, autism, atherosclerosis, etc. Reactive oxygen species, though, are essential as they are involved in the immune response against pathogens.
Examples of reactive oxygen species are peroxides, singlet oxygen, superoxide, and hydroxyl radical.