EXPLORING LIFESPAN DEVELOPMENT

Exploring Lifespan Development



Exploring Lifespan Development

Atoms are most stable in the ground state. An atom is considered to be "ground" when every electron in the outermost shell has a complimentary electron that spins in the opposite direction. By definition a free radical is any atom (e.g. oxygen, nitrogen) with at least one unpaired electron in the outermost shell, and is capable of independent existence. A free radical is easily formed when a covalent bond between entities is broken and one electron remains with each newly formed atom. Free radicals are highly reactive due to the presence of unpaired electron(s). The following literature review addresses only radicals with an oxygen center. Any free radical involving oxygen can be referred to as reactive oxygen species (ROS). Oxygen centered free radicals contain two unpaired electrons in the outer shell. When free radicals steal an electron from a surrounding compound or molecule a new free radical is formed in its place. In turn the newly formed radical then looks to return to its ground state by stealing electrons with antiparallel spins from cellular structures or molecules. Thus the chain reaction continues and can be "thousand of events long.” The electron transport chain (ETC), which is found in the inner mitochondrial membrane, utilizes oxygen to generate energy in the form of adenosine triphosphate (ATP). Oxygen acts as the terminal electron acceptor within the ETC. The literature suggests that anywhere from 2 to 5% of the total oxygen intake during both rest and exercise have the ability to form the highly damaging superoxide radical via electron escape. During exercise oxygen consumption increases 10 to 20 fold to 35-70 ml/kg/min. In turn, electron escape from the ETC is further enhanced. Thus, when calculated, .6 to 3.5 ml/kg/min of the total oxygen intake during exercise have the ability to form free radicals. Electrons appear to escape from the ETS at the ubiqunone-cytochrome c level.

There are numerous types of free radicals that can be formed within the body. This web site is only concerned with the oxygen centered free radicals or ROS. The most common ROS include: the superoxide anion (O2-), the hydroxyl radical (OH ·), singlet oxygen (1O2 ), and hydrogen peroxide (H2O2) Superoxide anions are formed when oxygen (O2) acquires an additional electron, leaving the molecule with only one unpaired electron. Within the mitochondria O2- · is continuously being formed. The rate of formation depends on the amount of oxygen flowing through the mitochondria at any given time. Hydroxyl radicals are short-lived, but the most damaging radicals within the body. This type of free radical can be formed from O2- and H2O2 via the Harber-Weiss reaction. The interaction of copper or iron and H2O2 also produce OH · as first observed by Fenton. These reactions are significant as the substrates are found within the body and could easily interact. Hydrogen peroxide is produced in vivo by many reactions. Hydrogen peroxide is unique in that it can be converted to the highly damaging hydroxyl radical or ...