Reactive oxygen species describes a group of chemically active molecules that contain oxygen. They are chemically active in that they are charged, in other words their electrical charge is not neutral.
This comes from their outer electron shell lacking an electron, that is, it has an unpaired electron giving it a net positive charge. The effect is that the molecule is looking to acquire another electron from a neighboring molecule in order to stabilize its outer electron shell.
"Free radical" is another term applied to reactive oxygen species.
These free radicals can be categorized into five types, four of which are reactive oxygen species while the fifth is a reactive nitrogen species in that it is a nitrogen derivative instead of oxygen. The five categories of free radicals are:
Reactive oxygen species are included here to show that there are many different type of free radicals and no single antioxidant is capable of dealing with all of them.
Each of the ROS and RNS listed above have different constructions, operate differently and can inflict different types of damage to cells.
Consider that the superoxide ion is very toxic and is used by the immune system to dispose of foreign microorganisms however, with its extra electron, it can damage a cells mitochondria and/or DNA.
It can be detoxified or neutralized by enzymatic reaction with superoxide dismutase.
Hydroxyl radicals are highly reactive oxygen specific radicals with such a short life that it is extremely damaging to any type of large organic molecule.
The singlet oxygen is unique in that it can persist for an hour or more and is linked to the oxidation of low density lipoproteins (LDL).
Hydrogen peroxide is a natural byproduct of metabolism and quickly and easily converts to an oxidizing hydroxide free radical.
The good news is that organisms that live in an oxygenated environment, like us humans, have natural enzymes called peroxidases that cause hydrogen peroxide to decompose into water and oxygen.
Reactive nitrogen species is only mentioned because it acts with reactive oxygen species to damage cells.
All of the reactive oxygen species have the ability to alter DNA with resulting problems in DNA replication and repair. When ROS oxidizes DNA bases, mutations in duplications can occur with implications for cancer, Parkinson's, Alzheimer's and about 60 other diseases.
Cell death can occur when lipids in cell membranes become oxidized to the extent that the membrane is weakened, leaks occur and it eventually disintegrates.
We don't want an excess of free radicals forming in our body; these guys are bad actors and that is where antioxidants come into the picture.
At the risk of sounding repetitive, an antioxidant molecule prevents the oxidation of other molecules by donating one of its electrons to restore charge neutrality to the free radical.
The wonderful thing about antioxidants is that they are not only electron donors; they can give up an electron without becoming free radicals themselves, thus breaking the chain reaction.
Since it is well known that our body’s natural antioxidant protection declines with age and, for that reason several antioxidants are touted as a means of forestalling the aging process.
It is extremely important to maintain glutathione levels as we age since this is the one antioxidant that can take care of all types of reactive oxygen species.
Some of the most studied natural antioxidants that our body uses to stop the chain reaction of reactive oxidative species are vitamins A, C and E plus the carotenes, flavonoids and amino acids such as alpha-lipoic acid.
Most of these are essential nutrients meaning that we have to get them from food; our body doesn't make them naturally. This makes it ever more important to consume organic, vine or field ripened produce from local markets where possible.
Antioxidants are either non-enzymatic or enzymatic where the first works by interrupting the chain reaction and the latter by breaking down free radicals and removing them. Enzymatic antioxidants are covered in a separate section.
One other categorization of antioxidants is whether they are water soluble or fat soluble.
It matters greatly because the fluid in our cells and the fluid surrounding them is mostly water. On the other hand the cell membrane is mostly composed of lipids; i.e., fat.
Since reactive oxygen species can attack both the cells and their membranes, we need antioxidants that can work in both environments.
Water soluble antioxidants include vitamin C, the polyphenols and glutathione and vitamins A, E, cartenoids and alpha lipoic acid are fat soluble.
Before moving on to look at individual antioxidants, a couple of good source references provided by Dr. Mercola's blog of May 16, 2011 are shown below.