Aggressive molecules containing oxygen that can damage cellular components and cause oxidative stress.Reactive oxygen species (ROS) are oxygen-containing molecules that are chemically eager to react with other molecules, often stealing electrons from them in the process. The family includes free radicals such as superoxide and the hydroxyl radical, as well as non-radical but still reactive molecules like hydrogen peroxide. Because oxygen is central to how most cells generate energy, ROS are produced constantly as by-products of normal metabolism, especially in the mitochondria. The same reactivity that makes them useful also makes them dangerous: they can oxidize lipids in cell membranes, damage proteins, and attack DNA, a condition known as oxidative stress. Cells therefore maintain a balance between ROS production and a set of defensive molecules and enzymes that neutralize them. When that balance tips toward excess ROS, the resulting damage accumulates and is implicated in aging and in many diseases, including cancer, neurodegeneration, and cardiovascular disorders. At the same time, ROS are not purely destructive; they serve as signaling molecules in immune responses and in the regulation of cell growth and death.
The concept emerged in the mid-20th century. In 1954, Rebeca Gerschman and colleagues proposed that oxygen toxicity and radiation damage share a common mechanism involving oxidizing radicals. The term "reactive oxygen species" came into wide use later, and the role of these molecules in biology was further established by the discovery of superoxide dismutase by Joe McCord and Irwin Fridovich in 1969.
The core mechanism is electron transfer. Molecular oxygen, when partially reduced, forms superoxide; further reactions produce hydrogen peroxide and, in the presence of certain metal ions, the highly reactive hydroxyl radical. These species react with whatever is nearby, abstracting electrons from lipids, proteins, or nucleic acids and setting off chain reactions, such as lipid peroxidation in membranes. Cells counter this with antioxidant systems: enzymes like superoxide dismutase and catalase convert reactive species into harmless water and oxygen, while small molecules such as glutathione donate electrons to neutralize radicals. The outcome for the cell depends on the local balance between production and removal.
ROS are measured in biomedical research as markers of oxidative stress, and antioxidants that modulate ROS levels are studied for treating inflammatory and age-related diseases. In industry, controlled ROS generation is used for sterilization and for degrading pollutants in wastewater.
Some immune cells deliberately produce a burst of reactive oxygen species to kill engulfed bacteria, turning these damaging molecules into a weapon.