The environment we inhabit contains around 20 % oxygen, and animals and plant cells require this oxygen to respire. During respiration, energy in the form of glucose is broken down to simpler chemicals, ultimately leading to the production of carbon dioxide and water. This final step in the formation of energy requires molecular oxygen (O2). However, oxygen is not a benign chemical, because it readily reacts with many other chemicals causing oxidation. These oxidation reactions are generally damaging, and we can see oxidation in the environment around us in the formation of rust, the formation of rancid fats and the general deterioration that occurs to rubber as it ages. As in the environment oxidation occurs in our tissues, because we utilise oxygen and transport it around our bodies to provide energy through respiration. In this process the oxygen comes into contact with cellular components and here oxidation can occur leading to tissue damage. If left uncontrolled, oxidation leads to the formation of disease.
Oxidation is damaging because it causes the formation of free radicals. Free radicals are chemicals, that have unpaired electrons brought about through oxidation. These unpaired electrons make the molecule highly reactive, and this reactivity causes the molecule to react with other molecules, in turn leaving the second molecule with unpaired electrons. This process is propagated, producing a free radical chain reaction, which damages cellular components, ultimately leading to disease. Free radicals can also be generated from chemicals other than molecular oxygen, with many pollutants, toxins and poisons in the environment being able to generate free radicals through oxidation reactions. Controlling free radicals is therefore pivotal to health, and the cells of the body have a number of chemicals that can prevent free radical chain reactions from forming. These chemicals are antioxidants, and they works by donating electrons to the free radical, thus inhibiting their reactivity by repairing up the unpaired electrons.
The antioxidants synthesised naturally by the cells to protect from free radicals are enzymes such as glutathione peroxidase and superoxide dismutase. These enzymes require minerals such as selenium, zinc or copper to function, and deficiencies of these minerals reduces cellular antioxidant levels, decreasing the ability of cells to inhibit free radical chain reactions. Antioxidants can also be vitamins, and both vitamin C and vitamin E are dietary antioxidants that support the enzymatic antioxidants at preventing free radical damage. Other non-vitamin dietary antioxidants are obtained from plant foods, and plants contain a very large number of chemicals that are absorbed in humans and then act as antioxidants in tissues. The most well know group of these is called the polyphenols, and includes chemicals including flavonoids, stilbenes, terpenes and chalcones. If the antioxidant defences of the cells, become overwhelmed by free radicals, the situation of uncontrolled free radical generation is termed oxidative stress.
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