The term "redox" is short for the chemical process known as "reduction-oxidation." It refers to the transfer of electrons that occurs during many chemical reactions. Electron transfer is important to the life of a cell. During cellular respiration, there is a relocation of electrons. This relocation results in the release of energy that is stored in food molecules, and the released energy is used to synthesize ATP. Remember the first law of thermodynamics which describes the conversion of energy; you can't "make" energy, but energy can be transferred from one molecule to another. Redox is one way that this is accomplished.
During many cellular reactions, energy changes are due to the movement of electrons (molecules either gain or lose electrons). A molecule that gains an electron is reduced, meaning that there has been a "reduction" in its positive charge. This gain in electrons is termed reduction . A molecule that loses an electron is oxidized, and this loss of electrons is termed oxidation . You can remember this somewhat confusing terminology with the mnemonic "OiL RiG", where (O)xidized species (L)ose electrons and (R)educed species (G)ain electrons.
The general equation of a redox reaction is:
In this equation, molecule "X" is oxidized and loses an electron (and energy), whereas molecule "Y" is reduced and gains an electron (and energy).
Some molecules have a very high affinity for electrons, therefore, they are more likely to oxidize other molecules. The affinity for electrons is termed electronegativity . The more electronegative a molecule is, the more likely it will become reduced by the addition of electrons in a chemical reaction.
Importantly, not only do electrons move from one molecule to another during a redox reaction, so does energy. In other words, a molecule that is oxidized loses energy, whereas a molecule that is reduced gains energy.
Methane (CH4 ; also known as natural gas) combustion releases energy, CO2 , and H2 O as final products. This occurs in the presence of oxygen (O2 ; which is highly electronegative) every time one ignites a gas stove in a house supplied with natural gas. (This example was also used in the tutorial Carbon and Life)
This figure illustrates the conversion of methane gas into CO2 , H2 O, and energy. Note that while electrons are transferred from methane to oxygen, energy is released that can do work (the reaction is exergonic). In the process of redox, methane is converted into carbon dioxide (which contains an oxidized form of carbon) and water (which contains a reduced form of oxygen). The electrons that are associated with oxygen in the water molecule have much less energy than they had when associated with carbon in methane; this energy difference is released as heat.
While heat is useful for cooking, it is not very useful for biological reactions. During redox reactions that occur within your body, a high proportion of the energy that is released is moved into other forms of chemical energy. However, heat is released as a by-product of these reactions because no reaction is 100% efficient.