Metabolism is a balance of oxidation and reduction. Oxidative processes have to be contained through defence mechanisms which are developed through an anti-oxidant system which nullifies excess free radicals produced by oxidative processes. The anti-oxidant system includes mineral-dependent enzymes and small molecules, usually vitamins, which act as scavengers of reactive oxygen species. The enzymes include the selenium-dependent free radical scavenger, glutathione peroxidase. The small molecular weight molecules include the water-soluble ascorbic acid, glutathione and uric acid and the lipid-soluble carotenoids and vitamin E.

If there is a compound where A and B are two atoms covalently bonded. xx represent the electron pair. Homolytic fission can be written as: AxxB® Ax + Bx Ax is an A-radical, often written as A∙ and Bx is a B-radical (B∙). Homolytic fission of one covalent bond in a water molecule will produce a hydrogen radical (H∙) and a hydroxyl radical (OH∙). A contrast to homolytic fission is heterolytic fission when one atom receives both electrons when a covalent bond breaks, i.e. AxxB® Axx– + B+ This extra electron gives A a negative charge and B is left with a positive charge. Heterolytic fission of water gives a hydrogen ion H+ and a hydroxyl ion OH–.

Radicals are groups of atoms which behave as a unit. A free radical is any chemical type capable of independent existence that contains one or more unpaired electrons. An unpaired electron is one that occupies an atomic or molecular orbital by itself. Radicals can easily be formed by homolytic fission when a covalent bond is split, when one electron from each of the pair shared remains with each atom.

1. Metabolism is a balance of oxidation and reduction. Oxidative processes have to be contained through defence mechanisms, utilising an antioxidant system which nullifies excess free radicals engendered by oxidative processes. The antioxidant system includes mineral-dependent enzymes and small molecules (usually vitamins) which act as scavengers of reactive oxygen species.

2. A free radical is any species capable of independent existence that contains one or more unpaired electrons. An unpaired electron is one that occupies an atomic or molecular orbital by itself.

3. Oxidation is the loss of electrons by an atom or molecule, e.g. the conversion of a sodium atom to the ion Na⁺. Reduction is the gain of electrons by an atom or molecule, e.g. the conversion of a chlorine atom to the ion Cl^–. An oxidizing agent absorbs electrons from the molecule it oxidizes whereas a reducing agent is an electron donor.

4. The major constituent of living cells is water. Exposure of such water to ionizing radiation results in hydroxyl radical production and possibly damage to cellular DNA and to membranes.

5. It is important for cells to control the amount of hydrogen peroxide in the cell. This may be achieved enzymatically, e.g. catalase, peroxidases, superoxide dismutase and copper-zinc and manganese enzymes.

6. There is protection against oxygen radicals by small molecules, e.g. ascorbic acid, glutathione and uric acid. Lipid peroxidation is the oxidative deterioration of polyunsaturated lipids. Superoxidation of a polyunsaturated fatty acid involves the removal of a hydrogen ion from a methylene (-CH₂-) group. Protection is given by vitamin E and glutathione peroxidase.

7. It has been suggested that in tissue deprived of oxygen there is a rapid conversion of xanthine dehydrogenase to oxidase activity. During ischaemia there is degradation of ATP in the ischaemic cells and accumulation of hypoxanthine. However, when oxygen is restored there is reperfusion damage. The accumulated hypoxanthine is oxidized by xanthine oxidase and the excessive O₂^– causes further damage.