A free radical is any atom or molecule that contains one or more unpaired electrons in an orbital. The degree of chemical reactivity depends on the localization of unpaired electrons. Free radicals are extremely reactive, and they can either donate or accept an electron from other molecules. Free radicals that include oxygen radicals and derivatives of oxygen are reactive oxygen species (ROS). Likewise, reactive nitrogen species (RNS) are nitric oxide-derived compounds. ROS/RNS include oxygen/nitrogen free radicals and non-radicals that are easily converted into radicals. Mitochondria are a main endogenous source of free radicals in cells and consequently are exposed to oxidative-nitrosative damage. Electron transfer in the electron transfer-pathway (ET-pathway) is not perfect, leading an electron leakage. This electron leakage permits the formation of ROS such as superoxide anion (O2•−), hydrogen peroxide (H2O2) and the hydroxyl radical (HO•).
Reference: Halliwell et al 2001
MitoPedia topics: Substrate and metabolite