Free-radical theory of aging.

http://en.wikipedia.org/wiki/Free-radical_theory_of_aging

The free radical theory of aging (FRTA) states that organisms age because cells accumulate free radical damage over time.A free radical is any atom or molecule that has a single unpaired electron in an outer shell. While a few free radicals such as melanin are not chemically reactive, most biologically-relevant free radicals are highly reactive.For most biological structures, free radical damage is closely associated with oxidative damage. Antioxidants are reducing agents, and limit oxidative damage to biological structures by passivating them from free radicals.

Strictly speaking, the free radical theory is only concerned with free radicals such as superoxide ( O₂^- ), but it has since been expanded to encompass oxidative damage from other reactive oxygen species such as hydrogen peroxide (H₂O₂), or peroxynitrite (OONO^-).

Denham Harman first proposed the free radical theory of aging in the 1950s, and in the 1970s extended the idea to implicate mitochondrial production of reactive oxygen species.

Processes

In chemistry, a free radical is any atom, molecule, or ion with an unpaired valence electron

Free radicals are atoms or molecules containing unpaired electrons. Electrons normally exist in pairs in specific orbitals in atoms or molecules.Free radicals, which contain only a single electron in any orbital, are usually unstable toward losing or picking up an extra electron, so that all electrons in the atom or molecule will be paired.

Note that the unpaired electron does not imply charge - free radicals can be positively charged, negatively charged, or neutral.

Damage occurs when the free radical encounters another molecule and seeks to find another electron to pair its unpaired electron. The free radical often pulls an electron off a neighboring molecule, causing the affected molecule to become a free radical itself. The new free radical can then pull an electron off the next molecule, and a chemical chain reaction of radical production occurs.The free radicals produced in such reactions often terminate by removing an electron from a molecule which becomes changed or cannot function without it, especially in biology. Such an event causes damage to the molecule, and thus to the cell that contains it (since the molecule often becomes dysfunctional).

The chain reaction caused by free radicals can lead to cross-linking of atomic structures. In cases where the free radical-induced chain reaction involves base pair molecules in a strand of DNA, the DNA can become cross-linked.

DNA cross-linking can in turn lead to various effects of aging, especially cancer.