Reactive oxygen species, ROS, include oxygen ions, free radicals and peroxides of both organic and nonorganic origin. As a rule those are small molecules that possess a very high ability to interact with other substances (to oxidize them) due to the presence of an unpaired electron in the outer shell.
ROS are known to present serious danger for living cells and whole organisms (even though under certain conditions they can also fulfill some important cell functions, e.g. participate in regulation).
The primary forms of ROS are superoxide (O2-.) and its derivative hydrogen peroxide (H2O2). And even though a number of enzymes responsible for transformation of O2 into the primary forms of ROS were discovered in living cells, all of them proved to produce far less ROS than the respiratory chain of the inner mitochondrial membrane. Mitochondria of an adult absorb about 400 liters of oxygen per day converting it to water in the process of four- electron reduction. At the same time if even 0,1% of this oxygen reduction takes place in a chemically more simple single electron manner, it will result in 0,4 liters O2-., and this amount proves to exceed substantially all the other possibilities of ROS generation mechanisms. Experimental data prove these calculations to be true. In other words mitochondria constantly produce the amount of ROS sufficient to oxidize cell DNA, proteins and lipids thus causing their damage.