Carbon vs Silicon

Sunday the 11th, and instead of being at home watching the usual rounds of Sunday night TV, I found my self lying on a rug on top of “old harry’s rocks” watching what seemed like hundreds of shooting stars come down around me. Although the cosmos never cease to fascinate me, as humans we seem to spend a lot of time looking up and wondering about the vast and unexplored expanse around us. Star wars, Star trek, cowboy’s vs aliens- we just cannot stop wondering whether there may be something out there and it is this curiosity that drives people to speculate about whether alien life can possibly be able to exist. Life on earth is mainly attributed to carbon (an element with great but seemingly improbable abundance on earth) and it is the next element down its column in the periodic table that causes all the fuss- Silicon.

Carbon and Silicon are both members of column 4 on the periodic table and share a lot of characteristics. Already, silicon is found to have been part of life forms such as sea urchins (silicon is an important component in their spines) and certain forms of protozoa (where silicon is used as an exoskeleton). It is also thought that theoretically the neurons in our brains can be replaced with silicon transistors, so creating a “Silicon brain”. Silicon is already an integral part of circuits and computer software, so showing its capability to do a job so very intricate and delicate.

However silicon lacks the versatility that carbon has, whilst carbon is agile and can form many different structures (sugars, hormones, DNA etc) , element 14 is the “elephant on the dance floor” as it were, and is cumbersome and will not form half the structures carbon will. Part of the problem is that Silicon has a whole eight more electrons than carbon (an extra shell) meaning that it is bigger and unable to form double bonds. Double bonds are an essential for life, as a way of storing vast amounts of energy and an integral component of many biological structures,  life as we know it simply couldn’t go on without it.

Not only this but Silicon lacks certain “chilarity/ handedness”. As Pasteur first discovered in his tartaric acid experiments, mostly every biological molecule has handedness and it is certain types of handedness (left handed or right handed) that help biological molecules recognize substrates and so react in the right way. This means that Silicon working in the body and reacting chemically with other substances would be very difficult to achieve and only extreme or impossible conditions would have to be present for this to happen at all.

SiO2 is also a problem. Unlike carbon dioxide, Silicon dioxide is a solid at room temperature therefore creating great difficulty in achieving any sort of gas exchange in a hypothetical body. Solids do not flow like liquids or gasses and so removing and taking in SI or SIO2 in anyway would be extremely hard. SiO2 also doesn’t dissolve in water (a particularly abundant substance) and there would be other problems in transporting the silicon around the body- what would the alien use for blood? for body fluids?

Silicon structures are unstable and react easily- in fact the largest Silicon molecule ever observed had only six atoms whilst carbon molecules can contain vast amounts more. Silicon is also much less abundant in the universe.

So unfortunately Silicon seems highly improbable for alternative life forms and although we can learn a lot about our elements by looking upwards, we may never be sure whether any other planet (no matter how extreme or abstract the conditions) could balance such delicate life  as ours.


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