Radiation may cause changes in complex molecular systems, such as living cells, in two ways.
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There are three important mechanisms by which radiation, both particulate and electromagnetic can directly affect biological systems.
1. ThermalizationThis is a process whereby molecules are non-destructively agitated by incident radiation and in the process they acquire additional kinetic energy. This manifests itself as an increase in the temperature of the molecular system. Of course if the temperature of the material becomes too high, complex organic molecules break apart (i.e. the material gets "cooked").It is interesting to note that the average kinetic energy of molecules in a cell (due to their thermal motion) is about 0.04eV, astonishingly close to the lower-limit of the molecular bond strength of some organic molecules. 2. ExcitationThis is a process whereby the incident radiation causes bound electrons to be "knocked" free from their parent atoms or molecules. These free electrons are then available to interact with other atoms and molecules within the irradiated system. Generally however the free electrons are quickly captured and their energy added to the overall thermal energy of the system with little or no other effect.3. IonizationThis is a process whereby molecules absorb sufficient energy from particle or photon radiation to break their molecular bonds. This causes the direct modification or destruction of complex molecules.Covalent bonds and ionic bonds, the bonds between the atoms that form simple inorganic molecules such as sodium chloride (common table salt) have binding energies of about 2 to 5eV, about one half the strength of the ionizing energies of individual atoms. Organic molecules, many of which are important in cell biology, have bond energies of about 0.04 to 0.3eV and are typically referred to as "weak bonds". |
| The most abundant substance in cells is water. Because of its abundance, it is the ubiquitous water molecule that absorbs the major fraction of all incident radiation.
Low energy radiation is simply dissipated by the thermal agitation of the molecule. In other words, it increases the molecule's kinetic energy slightly (its thermal energy increases). High energy absorption however can cause the molecular bonds within the molecule to break, creating H+ and OH- ions within the cell. Usually these ions have low energy and quickly recombine to form water; but, at higher energies free radicals form; then hydrogen peroxide will form through a series of reactions which can be summarized as follows. The substance H2O2 is common hydrogen peroxide, a highly reactive oxidizing compound. It is this highly reactive compound which can "attack" other molecules such as DNA and other important molecules which regulate and control vital cellular functions. Many other reactions involving water molecules are known, the formation of hydrogen peroxide is merely one (although important) example. Although the cell is able to repair most molecular damage, excessive molecular damage can lead to cellular death or mutation. The interaction of water molecules with incident radiation is by no means the only reaction that occurs when organic molecules are irradiated. The study of radiation biochemistry is extremely complex and much has yet to learned about how the molecules within cells react when they absorb energy from incident radiation. |
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