A free radical is any type of molecule capable of independent existence, containing one or more electrons from the non satisfied valences which, consequently, contribute nothing to the molecule's internal linking. The free radicals interesting for vital phenomena are only those that contain oxygen. They result from oxidation or reduction, whereby electrons are transferred one by one, or from the rupture of a covalent link leaving two non mated electrons, each from a different atom. Free radicals are extremely reactive due to the electron tendency to mate, by being either given away by an appropriate donor or delivered to an adequate receptor. Whenever a free radical reacts with another molecule, a chain reaction is initiated. It will continue until two free radicals react between them, putting an end to the propagation by the creation of a link wherein each radical contributes one non mated electron. Free radicals have a rather brief range of existence, whose duration is directly proportional to their reactivity (Samson, 1999).
There are many free-radical sources, internal as well as external to the cell (environmental). Most are produced by metabolism itself, as a result of the normal functioning of enzymes or the electron-transfer process of the mitochondrial respiratory chain. In fact, the oxidants generated by mitochondria are the most important reason for the damage progressively accumulated in the aging organism (Ames, 1993). On the other hand, free radicals are involved in several degenerative diseases, including Parkinson and Alzheimer, cataracts, arteriosclerosis, and late diabetes. All this contributes to their bad image. However, one should be aware of their contribution to normal physiology, extremely important. For instance, they crucially intervene in the immune system and in the cerebral information processing. More generally, their participation is essential in the omnipresent redox processes of cellular regulation: donation of an electron (reducing reaction) and acceptance of one (oxidizing reaction).
The organism possesses mechanisms in the form of specialized enzymes and nutrients –such as vitamins C and E– which together constitute a defensive system against the attack of free radicals, avoiding chronic illnesses and alleviating the consequences of aging. When the weight of the internal and external environment becomes too heavy, the food's quality or its assimilation too poor, free radicals impose themselves, giving way to oxidant stress, senile weakening, and disease.