Free radicals in normal cell function

A free radical is any molecule that contains an atom with an unpaired electron. They are produced by oxidation/reduction reactions where there is transfer of single electrons only or when a covalent bond is broken leaving a single electron with each atom. Free radicals are highly reactive. The presence of the unpaired electron in the valence shell renders the atom highly unstable, such that either donation or receipt of an electron, as appropriate, is favoured to stabilize the shell.

Many free radicals are produced in cells during normal metabolism (Fig. 3.48.1). A particularly rich source is the electron transport system in mitochondria. Electrons that leak from the respiratory chain reduce molecular oxygen (O₂) directly to the superoxide anion (O₂^•−

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