When we think of oxygen, what usually comes to mind is that it gives us life. And indeed it does!. We can live without water or food for several days but not with oxygen. We can survive only for a few minutes without it. But on the contrary, it is also a main ingredient for aging.
Iron rusts when exposed to oxygen. That's why water pipes made of iron are coated with a special material to protect the metal from coming in contact with oxygen. Scrape off the protective coating and for several hours you will notice the metal's color will slowly turn to brown. That's rust forming.
Slice an apple and a few minutes later, the flesh will turn brown.
But you may ask, why doesn't the apple skin react with oxygen destructively? The molecules which compose the apple's skin have tightly bonded electrons. They don't react with oxygen the way the flesh inside does.
There are atoms whose electrons are not tightly bonded to their nucleus. When they come in contact with oxygen, one of their electrons can be taken away from them because oxygen is like a magnet for electrons.
Oxidation is the term used for atoms which loses or gives up an electron to oxygen. Depending on the type of atom, it may become reactive or unstable( a free radical). The tendency of the atom now is to regain its stability by 'stealing' an electron from a nearby atom. A chain reaction occurs and it only stops when an antioxidant is encountered.
Free radical chain reactions occur frequently within our bodies. Without antioxidants, our cells will die rapidly, DNA would mutate which could alter the natural growth of cells, or put simply we will 'rust'.
Antioxidants are the 'defenders' to the harmful effects of oxygen when it combines with other molecules.