Why Organic Chemistry?
The basic drive of life is to make more of itself. The chemical reactions required for the faithful propagation of a free-living organism necessarily require high degrees of specificity in the interactions of the molecules that carry out the propagation. Such specificity requires information, in the form of complex molecular structurelarge molecules. The molecules that serve terrestrial organisms typically are very large, proteins and RNAs with molecular weights of thousands to millions of daltons, or even larger as in the case of genetic DNA. It is predictable that life, wherever we encounter it, will be composed of macromolecules.
Only two of the natural atoms, carbon and silicon, are known to serve as the backbones of molecules sufficiently large to carry biological information. Thought on the chemistry of life generally has focused on carbon as unique (3). As the structural basis for life, one of carbon's important features is that unlike silicon it can readily engage in the formation of chemical bonds with many other atoms, thereby allowing for the chemical versatility required to conduct the reactions of biological metabolism and propagation. The various organic functional groups, composed of hydrogen, oxygen, nitrogen, phosphorus, sulfur, and a host of metals, such as iron, magnesium, and zinc, provide the enormous diversity of chemical reactions necessarily catalyzed by a living organism. Silicon, in contrast, interacts with only a few other atoms, and the large silicon molecules are monotonous compared with the combinatorial universe of organic macromolecules.
Life also must capture energy and transform that energy into the chemistry of replication. The electronic properties of carbon, unlike silicon, readily allow the formation of double or even triple bonds with other atoms. These chemical bonds allow the capture and delocalization of electronic energy. Some carbon-containing compounds, therefore, can be highly polarized and thereby capture "resonance energy" and transform this chemical energy to do work or to produce new chemicals in a catalytic manner. The potential polarizability of organic compounds also contributes to the specificity of intermolecular interactions, because ionic and van der Waals complementarities can shift to mesh with or to repulse one another. Finally, it is critical that organic reactions, in contrast to silicon-based reactions, are broadly amenable to aqueous conditions. Several of its properties indicate that water is likely to be the milieu for life anywhere in the universe (2).
The likelihood that life throughout the universe is probably carbon-based is encouraged by the fact that carbon is one of the most abundant of the higher elements. Astronomical studies find complex organic compounds strewn throughout interstellar space. Moreover, the common occurrence of carbonaceous meteorites testifies to an organic-rich origin for our own solar system. If life indeed depends on the properties of carbon, then life is expected to occur only in association with second- or later-generation stars. This is because carbon is formed only in the hearts of former stars, so far as we know.