Thirdly, by using oxygen as a kind of 'glue', both silicon and carbon atoms can form long chains called polymers. Two examples of this are the carbon-based poly-acetal, a kind of plastic, and silicon-based polymeric silicones, which we use for waterproofing and lubrication. It is carbon's ability to form long complex chains that led to the formation of DNA ... and it is entirely conceivable that silicon could evolve something similar. However, while there are marked similarites between the two elements (which is why they appear so close together on the Periodic Table), there are some major differences too.
Secondly, the chemistry of life is difficult (although not impossible) to visualise for a silicon-based life form. When carbon unites with oxygen (oxidises) during breathing, it forms carbon dioxide, a gas. We breathe it out and plants breathe it in. However, when silicon oxidises, it forms a solid called silicon dioxide. It's hard to imagine a creature that breathes in oxygen and breathes out something that is essentially sand. There would also be some 'disposal' issues for a silicon-based life form as it would excrete similar silica-based substances (it adds a whole new dimension to the expression 'Shi**ing a brick'!).
Thirdly, where would a silicon-based life form draw its energy from? All living things need a way to collect, store and utilise energy. Once absorbed or ingested, the energy must be released exactly where and when it is needed within the body. Otherwise, all of the energy might liberate its heat at once, incinerating the life-form. In a carbon-based life-form, storage takes the form of carbohydrates (the curse of us fatties). A carbon-based life-form 'burns' this fuel in controlled steps using speed regulators called enzymes. Carbohydrates (the clue is in the name) are carbon-based compounds that oxidise to form water and carbon dioxide, which are then exchanged with the air. Silicon doesn't form many compounds that will duplicate the function of enzymes so it's hard to imagine how a silicon-based living organism could function.
Of course, as we've so often said on this blog, it would be narrow-minded to assume that life can only exist in the so-called 'Goldilocks Zone' of liquid water, oxygen atmosphere and moderate temperature that we do.
The Web, a silicon-based artificially created life form used to trap the spaceship Liberator in Blake's 7Raymond Dessy, professor of Chemistry at Virginia Polytechnic Institute and State University has this to say:
'It is possible to think of micro- and nano-structures of silicon; solar-powered silicon forms for energy and sight; a silicone fluid that could carry oxidants to contracting muscle-like elements made of other silicones; skeletal materials of silicates; silicone membranes; and even cavities in silicate zeolites that have handedness. Some of these structures even look alive. But the chemistries needed to create a life-form are simply not there. The complex dance of life requires interlocking chains of reactions. And these reactions can only take place within a narrow range of temperatures and pH levels. Given such constraints, carbon can and silicon can't.'
And UK astronomer and science writer David Darling points out that:
'The absence of silicon-based biology, or even silicon-based prebiotic chemicals, is also suggested by astronomical evidence. Wherever astronomers have looked – in meteorites, in comets, in the atmospheres of the giant planets, in the interstellar medium, and in the outer layers of cool stars – they have found molecules of oxidized silicon (silicon dioxide and silicates) but no substances such as silanes or silicones which might be the precursors of a silicon biochemistry.'
A vacuum 'breathing' silicon life form (artist unknown)And I'll leave the final words to ATS and their speculative study of the possibility of silicon-based life:
'A silicon-based organism might live on a planet without oxygen. There is no good reason why an organism could not use another gas as a phosphorylation catalyst, like hydrogen, nirogen or other reactive gases. It is also possible that methane might replace phosphor in the metabolic pathway. This would allow totally different matabolisms which might not have the problem of producing a lot of solid waste. A major component of glass is silicon. It might be possible that an organism based on glass exists. An organism like that may get its energy from solar-cell like cells. Zeolites, microporous materials based on silicon, aluminium and oxygen, might play a role in these organisms.'
Then, of course, there's life based on nitrogen, phosphorous, arsenic, ammonia ...
Silicon-Based Life (SCIAM)
Silicon-Based Life (University of Winnipeg)
Silicon-Based Life (UCLA)