Oxidation is a chemical process in which an atom, ion or molecule loses electrons or increases its oxidation state, often accompanied by the removal of hydrogen or addition of oxygen.
Explanation
Oxidation and reduction are complementary parts of redox reactions. Although early definitions described oxidation as the combination of a substance with oxygen, modern chemistry defines it more broadly as the loss of electrons to an oxidizing agent. When one species is oxidized, another is reduced and accepts the electrons. Biological systems rely heavily on oxidation reactions to harvest and transfer energy. During catabolic processes such as glycolysis, the citric acid cycle and β‑oxidation, electrons are stripped from carbohydrates and fatty acids and transferred to electron carriers like NAD+ and FAD, converting them into NADH and FADH2. These reduced coenzymes are subsequently oxidized in the electron transport chain, with the released energy driving ATP synthesis via oxidative phosphorylation. Enzymes such as dehydrogenases facilitate these oxidations by coupling the removal of hydrogen atoms with electron transfer. Oxidation is also involved in anabolic processes like the formation of disulfide bonds in proteins and the detoxification of xenobiotics by cytochrome P450 enzymes. In the environment, chemolithotrophic bacteria obtain energy by oxidizing inorganic substrates including ammonia, nitrite, sulfur compounds or ferrous iron. Because oxidation reactions can generate reactive oxygen species, cells employ antioxidant systems to prevent oxidative damage.
Biological oxidation examples
In aerobic respiration, glucose is oxidized to carbon dioxide; electrons removed during glycolysis and the tricarboxylic acid cycle are transferred to NAD+ and FAD. Fatty acids undergo β‑oxidation in mitochondria or peroxisomes, yielding acetyl‑CoA and reduced coenzymes. Nitrifying bacteria such as Nitrosomonas europaea oxidize ammonia to nitrite, and Nitrobacter species oxidize nitrite to nitrate, playing key roles in the nitrogen cycle. Sulfur-oxidizing bacteria like Acidithiobacillus thiooxidans convert hydrogen sulfide or elemental sulfur to sulfate, while iron-oxidizing bacteria such as Acidithiobacillus ferrooxidans oxidize ferrous iron to ferric iron. In human physiology, alcohol dehydrogenase oxidizes ethanol to acetaldehyde, and monoamine oxidase oxidizes neurotransmitters. These examples illustrate the diversity of oxidation reactions in metabolism and ecological processes.
Oxidation is inseparable from reduction; together they drive the flow of electrons that powers life. Understanding oxidation helps explain energy generation, biosynthesis and the environmental cycling of elements.
Related Terms: reduction, redox reaction, oxidative phosphorylation, electron transport chain, reactive oxygen species