Oxidation [ok-si-dey-shuhn] is any chemical reaction that involves a loss of electrons. For example, when iron reacts with oxygen it forms a chemical called rust: the iron is oxidized (loses electrons) and the oxygen is reduced (gains electrons).

A reduction reaction always comes together with its opposite, the oxidation reaction, and together are called ‘redox’ (reduction and oxidation). Although oxidation reactions are commonly associated with the formation of oxides from oxygen molecules, these are only specific examples of a more general concept of reactions involving electron transfer.

The word ‘oxidation’ originally implied reaction with oxygen to form an oxide, since (di)oxygen was historically the first recognized oxidizing agent. Later, the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions. Ultimately, the meaning was generalized to include all processes involving loss of electrons. The word ‘reduction’ originally referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was ‘reduced’ to metal. Antoine Lavoisier (1743-1794) showed that this loss of weight was due to the loss of oxygen as a gas. Later, scientists realized that the metal atom gains electrons in this process. The meaning of reduction then became generalized to include all processes involving gain of electrons. Even though ‘reduction’ seems counter-intuitive when speaking of the gain of electrons, it might help to think of reduction as the loss of oxygen, which was its historical meaning.

Many important biological processes involve redox reactions. Cellular respiration, for instance, is the oxidation of glucose to carbon dioxide and the reduction of oxygen to water. Photosynthesis is the reaction in reverse (the reduction of carbon dioxide into sugars and the oxidation of water into molecular oxygen). Free radical reactions are redox reactions that occur as a part of homeostasis and killing microorganisms, where an electron detaches from a molecule and then reattaches almost instantaneously. Free radicals (molecules with an unpaired electron) are a part of redox molecules and can become harmful to the human body if they do not reattach to the redox molecule or an antioxidant (a molecule that inhibits oxidation reactions by donating an electron to the free radical). Unsatisfied free radicals can spur the mutation of cells they encounter and are thus causes of cancer.

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