Reactive oxygen species

Reactive oxygen species are oxygen-containing molecules that readily participate in chemical reactions and can modify proteins, lipids, and DNA. Cells produce them during metabolism and also use them in signaling and defense.

Superoxide: A negatively charged oxygen molecule with an unpaired electron, produced in several cellular reactions. It is a primary reactive oxygen species and a source of other oxidants.
Free radical: An atom or molecule containing an unpaired electron, often making it chemically reactive. Several reactive oxygen species are radicals, though not all are.
Oxidative stress: A state in which oxidant production exceeds the capacity of biological systems to control or repair resulting damage. It describes imbalance when reactive oxygen species overwhelm cellular defenses.
Reactive nitrogen species: Reactive molecules derived from nitrogen that participate in signaling and chemical damage. They are a related oxidant family, but their central atom and reactions differ.
Hydrogen peroxide: A relatively stable oxygen compound that can oxidize cellular targets and serve as a signaling molecule. Cells convert superoxide into this longer-lived species, which can cross membranes.
Oxidation–reduction reaction: A chemical reaction involving transfer of electrons between substances. Reactive oxygen species form and act through electron-transfer chemistry.
Respiratory burst: A rapid increase in oxygen consumption by immune cells that generates antimicrobial reactive oxygen species. Phagocytes use these oxidants to kill engulfed microbes.
Singlet oxygen: An electronically excited form of molecular oxygen with altered reactivity. It is a reactive oxygen species but is not a free radical.
Hydroxyl radical: A highly reactive radical containing one oxygen and one hydrogen atom. It reacts near its point of formation, causing severe local molecular damage.
Mitochondrial electron transport chain: A series of protein complexes that transfer electrons and help generate ATP in mitochondria. Electron leakage from this chain can reduce oxygen to superoxide.

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