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Free Radicals Generated in 3 Ways
There is not an absolute answer to origin of free radicals in the universe, just like it is hard to answer the origins of life, universe, etc.. But it is not hard to answer the question how free radicals are generated in the ecosystem on the earth. Chemically, there are about 3 ways free radicals are generated:
1) Photon-induced formation of free radicals
This is the major and more significant way of free radical generation than the other 2 ways to be discussed in the next. Free radicals are highly energetic chemical species in general, and require energy for their formation. The rich energy source for the free radical formation is the sun light – photons. The plant photosynthesis generates the most abundant free radicals – oxygen molecules via a variety of oxygen free radical intermediates. This way of free radical generation is crucial for the existence of human and other animal life on the earth. The sun light also causes diverse free radical formation in the atmosphere. The sun light can cause the split of oxygen molecule (diatomic molecule) into 2 oxygen atoms (or 2 radicals) in the upper atmosphere. Each oxygen free radical atom in the atmosphere may combine with an oxygen molecule to form ozone (another highly energetic chemical species). Ozone gas derived from oxygen free radicals forms an ozone layer in the upper atmosphere, protecting human life on the earth from the damages of UV light of the sun. How can one say that "free radicals are harmful byproducts of metabolism and exposure to environmental pollutants”?
2) Free radical chain reactions to new free radicals
In the human body, free radicals are generated from other existing free radicals (the oxygen molecule). The exception is that cancer radiation therapy also generates free radicals in the body. The oxygen molecule is converted to hydrogen peroxide in the electron transfer processes in the synthesis of ATP (see the next section). The resulting hydrogen peroxide contains a highly energetic oxygen-oxygen bond, and may be further converted to free radicals by homolytic cleavage of the oxygen-oxygen bond, or by another electron transfer. Oxygen free radicals and hydrogen peroxide in the human body are called reactive oxygen species (ROS) in biology.
3) Thermal chemical reactions generate free radicals
Free radicals can be generated at high temperature as they require high energy. An example is free radical formation when saturated alkanes are heated at high temperatures to break the carbon-carbon and/or carbon-hydrogen chemical bonds in an isolated vessel without oxygen. Free radicals are also rich as the reactive intermediate in combustion, which are, however, mainly from free radical chain reactions involving oxygen. Free radicals may be generated at low temperature or room temperature, only from chemicals that have a highly energetic chemical bond. In some chemical labs, some free radical initiators (e.g. peroxybenzoic acid) belong to this kind. An example of high energetic chemical bond is oxygen-oxygen bond, which can be broken easily to form 2 free radicals. Another example is diaza type compound: 2 nitrogen atoms are connected together within an organic molecule. Upon slight heating, the 2 nitrogen atoms are broken from the rest of the molecule, forming a nitrogen molecule and 2 free radicals within the rest of organic molecule(s). Thermal chemical reactions that generate free radicals include single electron oxidation or reduction. Imagine that 2 neutral molecules have a high energy collision. One electron may jump from one molecule to another because of a big difference of the electronic affinity between the two molecules. Oxygen molecule is typical of stronger affinity for electron, and tends to gain an electron from another molecule that holds electron very loosely.