2.16   Special Topic: Alkanes as Biomolecules

In truth, there really isn’t very much biochemistry of alkanes. The carbon–carbon and carbon–hydrogen bonds of alkanes are just too strong to enter easily into chemical reactions. These molecules are not totally inert, as we shall see shortly, but saturated alkane chains serve more as frameworks in our bodies—holding less strongly bonded atoms in the proper place for reactions to occur—than as reactive species themselves. However, as we have discussed earlier (p. 72), in the presence of oxygen and activating energy (a spark or a lighted match, for example), combustion occurs and heat is given off. We use that energy, that exothermicity of reaction, to warm our homes and propel our automobiles. Gasoline is largely a mixture of saturated hydrocarbons. Combustion is a very biological process. Life depends on it. So even though there isn’t very much biochemistry of alkanes, what little exists is the most important chemistry for life.

In the movie Mad Max: Beyond Thunderdome, the trading village Bartertown is powered by the methane produced by herds of pigs kept in the depths of Undertown. How did those piggies produce that methane? Bacteria in the stomachs of both pigs and cattle are capable of producing methane from plant material, and the belching and flatulence of those animals is a major source of planetary methane. Contrary to conventional wisdom, it is mostly belching that releases the methane.

Methane may also have been essential to the formation of life. The early atmosphere on Earth was relatively rich in methane and ammonia (as are the current atmospheres of most of the outer planets). Given an energy source such as ultraviolet radiation or lightning, methane and ammonia react to form hydrogen cyanide (HCN), a molecule that polymerizes to give adenine, a common component of ribonucleic acid (RNA) and other molecules of biological importance. In the presence of water and an energy source, methane and ammonia react to give amino acids, the building blocks of proteins.