Can Silicon serve as a replacement for Carbon based life?

So, unfortunately there is really no way I can relate this blog entry to Avatar. From what I remember from the movie, there was no mention of the chemical foundations of Pandora. The trees and plants did form these complex electrochemical connections that effectively acted as neurons, creating some sort of a planet-wide brain that had achieved sentience… which was incredibly awesome, but not really elaborated upon on a chemical basis.

Anyway, this topic of Silicon, as opposed to carbon-based life has major astrobiological implications. All known life on earth is built upon the sixth element, carbon, and carbon-based compounds. Most life forms contain a large majority of the elements hydrogen and oxygen (from water) as well. But the chemical processes that give something the status of “living” require carbon based molecular structures. The human body in particular uses some of carbon’s most unique chemical properties to function, mainly involving the storage of energy and creation of highly complex molecules and structures.

However, as illustrated with the creation of the creature called “Horta” in episode 26 of the original Star Trek series, its been speculated that another element (such as Silicon) could potentially replace carbon to form a very different biological basis of life. In fact, the first count of this speculation came from the German astrophysicist, Julius Scheiner, in 1891 when he suggested the suitability of silicon as a basis for life. Several years later the British chemist, James Reynolds, actually based his opening address to the British Association for the Advancement of Science on the fact that silicon has heat-stabilizing properties that could allow life to exist at extremely high temperatures (as we see with thermophiles on Earth).

Okay, so yeah people are and have been talking about silicon-based life for years, but when it comes down to the biochemistry it’s hard to justify as a potential reality. At first glance it is promising though. Silicon is the second-most common element in the Earth’s crust (far more abundant than carbon), though like carbon, is a “p-block” element of group IV of the periodic table suggesting significant similarities in their basic chemical qualities. For example, just as carbon commonly combines with four hydrogen atoms to form methane (CH4), silicon yields silane (SiH4), and in addition to these tendencies, both elements form long chains (polymers) in which they alternate with oxygen.

However, a significant difference in size and orbital energy between carbon and silicon may be the determining factor in the existence of silicon-based life. A silicon atom has eight more electrons than a carbon atom does, and its’ homogenous bond length is considerably greater than carbon’s: 235 pm in comparison to carbon’s 77 pm. Silicon’s larger electron cloud subjects its’ bonds to a greater magnitude of shielding, thus a silicon bond is generally weaker than a carbon bond. This difference alone is enough to explain why carbon makes life and silicon makes rocks, at least, in standard terrestrial conditions. Additionally, for these reasons, silicon is known to be largely incapable of forming as diverse a range of molecules as carbon under natural conditions, in addition to silicon’s strong affinity for oxygen molecules.

Silicon’s affinity for oxygen would be problematic for life formation, because when carbon is oxidized during the respiratory process of a terrestrial organism, it becomes gaseous carbon dioxide - a waste material that is easy for a creature to remove from its body. The oxidation of silicon, however, yields a solid because immediately upon formation, silicon dioxide organizes itself into a lattice in which each silicon atom is surrounded by four oxygens. Disposing of a substance such as this would pose a major respiratory challenge.

Additionally, in carbon-based biota, the basic energy storage compounds are carbohydrates in which the carbon atoms are linked by single bonds into a chain. A carbohydrate is oxidized to release energy (and the waste products water and carbon dioxide) in a series of controlled steps using enzymes. These enzymes are large, complex molecules (usually proteins), which catalyze specific reactions because of their shape and "handedness." A feature of carbon chemistry is that many of its compounds can take right and left forms, and it is this handedness (also called chirality) that gives enzymes their ability to recognize and regulate a huge variety of processes in the body. Silicon's failure to give rise to many compounds that display handedness makes it hard to see how it could serve as the basis for the many interconnected chains of reactions needed to support life.

So, ultimately it seems as though it’s pretty unlikely that silicon-based life could exist in a terrestrial setting in an Earth-defined biological sense. However, that hasn’t held back the imaginations of science fiction writers. As mentioned above, the Horta, a silicon-based life formed appearing in Star Trek existed on the planet, Janus IV. Apparently every 50,000 years, all the Horta die except for one individual who survives to look after the eggs of the next generation. Personally, the Horta seems to be the most unlikely form of a functional biological organism in terms of what we define “life” as on Earth. But see for yourself: http://www.youtube.com/watch?v=39roz9jQfzE.