Carbon and Life
It is hard to overstate the importance of carbon; its unique capacity for forming multiple bonds and chains at low energies makes life as we know it possible, and justifies an entire major branch of chemistry – organic chemistry – dedicated to its compounds. In fact, most of the compounds known to science are carbon compounds, often called organic compounds because it was in the context of biochemistry that they were first studied in depth.
What makes carbon so special is that every carbon atom is eager to bond with as many as four other atoms. This makes it possible for long chains and rings to be formed out of them, together with other atoms – almost always hydrogen, often oxygen, sometimes nitrogen, sulfur or halides. The study of these is the basis of organic chemistry; the compounds carbon forms with metals are generally considered inorganic.
Chains and rings are fundamental to the way carbon-based life forms – that is, all known life-forms – build themselves. Silicon is capable of forming the same sorts of bonds and structures, but opinion is divided on whether silicon-based life forms are a realistic prospect – in part because it needs higher energies to form them, and in part because whereas carbon dioxide (one of the main by-products of respiration, a process essential to all known life) is a gas and therefore easy to remove from the body, its counterpart silicon dioxide (silica) has an inconveniently high melting point, posing a serious waste disposal problem for any would-be silicon-based life form.
