Radicals are molecules, atoms or ions that have an unpaired (extra) electron in their outer (electron) shell. Unpaired electrons are highly reactive and play a part in many of many chemical reactions and biological processes.

Since these radicals are so reactive, they need to be tightly controlled in biologic environments, because excess radicals can cause damage to living cells. What kind of damage are we talking about? Damage to DNA that can cause mutations that can lead to cancer or damage to the lipid structure of cell membranes.

There are many different radicals, but for the scope of this article, we are just illustrating the role of oxygen radicals or oxygen reactive species. Oxygen usually has two unpaired electrons in the outer shell and that makes oxygen highly susceptible to radical formation.

Oxygen has an important role in energy production in our bodies and oxygen ultimately gets reduced (accepts electrons) to water inside the mitochondria. However, oxygen may form reactive oxygen species anywhere in the body by accepting just 1 electron and thus forming the radical superoxide. The superoxide radical can pick up 1 more unpaired electron and form peroxide (as in hydrogen-peroxide).

Reactive oxygen species are important in a number of biological pathways, such as those produced by phagocytes for the elimination of invading pathogens or produced by cells in the thyroid gland to produce thyroid hormone. But an excess of reactive oxygen species can be harmful to cells, in particular when these radicals damage cellular membranes (lipid peroxidation). In this process, the radical "attacks" the unsaturated fatty acids that are part of the lipid bi-layer of cell membranes. This can result in membrane rigidity, loss in permeability and diminished activity of membrane-bound enzymes.

How do our cells protect themselves from the actions of free radicals?
Enzymes such as superoxide dismutase, catalase and glutathione peroxidase are important enzymes that protect the cell from oxidative stress by converting the reactive oxygen species into less reactive molecules.

Additional protection comes from non-enzyme anti-oxidants, such as vitamin E, vitamin C and glutathione. Vitamin E is a lipid soluble antioxidant, which converts peroxyl radicals in cellular membranes. Vitamin C is also a strong antioxidant and also functions to recycle vitamin E radicals. Glutathione (a small peptide) can be easily oxidized by free radicals, but is regenerated quickly (in the liver).

We can obtain additional antioxidant defense from the food and fruits we eat. Many of these fruit antioxidants contain pigments and polyphenols that can easily neutralize radicals. Examples of potent and popular antioxidants are blueberries, acai, pomegranate, noni, goji and mangosteen fruit. Resveratrol (from the skin of red grapes) is also an important antioxidant.