Question

Assume that you are employed as a developmental biologist at NASA and the first returning mars explorer was found to have a small organism from mars onboard. The organism had the usual head-tail axis as well as dorsal-ventral polarity and what appeared to have right-left asymmetry. However upon analysis it was found to be composed of silicon based compounds and had no DNA, RNA or proteins at all. Discuss reasons why this organism would have similar developmental controls to those of earth’s carbon-based life and also discuss the opposing view as to why it would not.

Answer 1

ans:

Indeed, carbon and silicon share many characteristics. Each has a so-called valence of four--meaning that individual atoms make four bonds with other elements in forming chemical compounds. Each element bonds to oxygen. Each forms long chains, called polymers, in which it alternates with oxygen. In the simplest case, carbon yields a polymer called poly-acetal, a plastic used in synthetic fibers and equipment. Silicon yields polymeric silicones, which we use to waterproof cloth or lubricate metal and plastic parts.

But when carbon oxidizes--or unites with oxygen say, during burning--it becomes the gas carbon dioxide; silicon oxidizes to the solid silicon dioxide, called silica. The fact that silicon oxidizes to a solid is one basic reason as to why it cannot support life. Silica, or sand is a solid because silicon likes oxygen all too well, and the silicon dioxide forms a lattice in which one silicon atom is surrounded by four oxygen atoms. Silicate compounds that have SiO4-4 units also exist in such minerals as feldspars, micas, zeolites or talcs. And these solid systems pose disposal problems for a living system.

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.

There is one thing silicon can do. Life on earth is predominantly made up of right-hand carbohydrates and left-hand amino acids. Why do they not have the opposite handedness, or both have the same? Many chemists believe that the first "handed" carbon compounds formed in a "soupy" rock pool having a "handed" silica surface. And the handedness of this surface encouraged the creation of those carbon compounds now preferred in Earth's life-forms.