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The superoxide ion is an anion with an unpaired electron on one of its oxygen atom. Because of the unpaired electron, the superoxide ion is very reactive.

Superoxide is formed in cells during the electron transport chain. When formed in the cell, the superoxide ion attacks organic molecules, such as DNA, proteins and lipids, damaging the cell. To protect the cell from the superoxide, superoxide dismutase is produced to decompose the superoxide ion into hydrogen peroxide and oxygen. However, hydrogen peroxide is still toxic to the cell, so catalase is produced to decompose the hydrogen peroxide into oxygen and water.