Free radicals, atoms with unpaired electrons, cause damage to DNA and cell membranes, leading to cancer, aging, and disease. Mitochondrial DNA is particularly vulnerable. Antioxidants can prevent damage by donating electrons. Vitamin A, E, C, and selenium are important antioxidants that must be obtained through diet. Sunlight and pollutants also create free radicals.
When oxygen reacts chemically with other atoms, the interaction produces free radicals, which are atoms containing an unpaired electron in their outer shell. Unstable free radicals react rapidly with other compounds, trying to steal an electron. When the molecule under attack loses its electron, it develops itself into a free radical, setting off a chain reaction. The main threat from free radicals comes from the damage they inflict on cellular deoxyribonucleic acid (DNA) or cell membranes. Free radical damage, which contributes to cancer, aging and a range of diseases, can potentially be prevented with antioxidants, which are chemicals that easily donate electrons to free radicals.
Mitochondria are small cellular organelles that generate useful chemical energy for cells by moving electrons between molecules. Oxygen occupies the final position in the electron transport chain, and a faulty interaction between an electron and oxygen generates the radical form of oxygen, the most common free radical in living systems. The main mechanism of oxygen radical damage in the cell is the mutation of the cellular DNA. While the repair mechanisms in the cell can repair nuclear DNA, mitochondrial DNA (mtDNA) cannot be easily restored. The cumulative damage to mtDNA accumulates over time, resulting in extensive mitochondrial dysfunction, a decline in cellular energy supplies, and ultimate cell death.
Changes to the source molecules due to sunlight create more free radicals. In the lower atmosphere, the most notable examples are the light-induced dissociation of nitrogen dioxide, which produces oxygen and nitric oxide, a key ingredient in smog, and the photodissociation of ozone to form an excited oxygen atom. An important source of radicals is the dissociation of chlorofluorocarbons by ultraviolet radiation or reactions with other stratospheric constituents. These free radicals destroy the ozone layer.
Free radical damage to DNA leads to mutations that cause malignancy. Atherosclerosis, alcohol-related liver damage, and emphysema from cigarette smoking are all attributed to free radical-induced oxidation of many of the body’s chemicals. Free radicals can cause schizophrenia, Alzheimer’s disease, Parkinson’s disease and deafness. Another example of free radical damage is skin damage that occurs with excessive sun exposure. The “free radical theory of aging” proposes that free radicals trigger the aging process itself.
Oxidation is the process of free radical damage caused by oxygen radical. Antioxidants are molecules that can safely donate electrons to free radicals, interrupting the oxidative chain reaction before critical molecules are damaged. Although different enzyme systems in the body scavenge free radicals, major vitamin or mineral antioxidants include vitamin A or beta-carotene, vitamin E, vitamin C, and selenium, which is a trace metal. The body does not produce these nutrients so they must be obtained as part of a well balanced diet.
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