Raymond Dessy is a professor of Chemistry at Virginia Polytechnic Institute and State University in Blacksburg, Va. Here is his answer.
Group IV of the Periodic Table of the Elements contains carbon (C), silicon (Si) and several heavy metals. Carbon, of course, is the building block of life as we know it. So is it possible that a planet exists in some other solar system where silicon substitutes for carbon? Several science fiction stories feature silicon-based life-forms--sentient crystals, gruesome golden grains of sand and even a creature whose spoor or scat was bricks of silica left behind. The novellas are good reading, but there are a few problems with the chemistry.
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Image: HONG YANG, University of
Toronto CRYSTALLINE CREATURES? Silicon can grow into a number of lifelike
structures, but its chemistry makes it unlikely that it could be the basis for alien life-forms. |
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.
Also consider that a life-form needs some way to collect, store and utilize energy. The energy must come from the environment. Once absorbed or ingested, the energy must be released exactly where and when it is needed. Otherwise, all of the energy might liberate its heat at once, incinerating the life-form. In a carbon-based world, the basic storage element is a carbohydrate having the formula Cx(HOH)y. This carbohydrate oxidizes to water and carbon dioxide, which are then exchanged with the air; the carbons are connected by single bonds into a chain, a process called catenation. A carbon-based life-form "burns" this fuel in controlled steps using speed regulators called enzymes.
These large, complicated molecules do their job with great precision only because they have a property called "handedness." When any one enzyme "mates" with compounds it is helping to react, the two molecular shapes fit together like a lock and key, or a shake of hands. In fact, many carbon-based molecules take advantage of right and left-hand forms. For instance, nature chose the same stable six-carbon carbohydrate to store energy both in our livers (in the form of the polymer called glycogen) and in trees (in the form of the polymer cellulose).
Glycogen and cellulose differ mainly in the handedness of a single carbon atom, which forms when the carbohydrate polymerizes, or forms a chain. Cellulose has the most stable form of the two possibilities; glycogen is the next most stable. Because humans don't have enzymes to break cellulose down into its basic carbohydrate, we cannot utilize it as food. But many lower life-forms, such as bacteria, can.
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2 Comments
Add Commentso sense silica is a solid and crystal that exsplains why the martian manhunter can shapeshift cause crystals has that light thing going n can bend light and plus sence silicon or silica cant transport energy that well and life will exsplode. i assume that due to the unknown complications of life and sence mars is the hot or fire planet thay probly dont eat or is self sustained like similar to the marstian manhunter .or is body leaks heat on its own or something to survive. I think when looking at life we need to understand that life can make it and when somethings impossible it makes it possible. All we have to do is make substitutions or imagine around that impossibility thats what sci - fi is for.
Reply | Report Abuse | Link to thisso sense silica is a solid and crystal that exsplains why the martian manhunter can shapeshift cause crystals has that light thing going n can bend light and plus sence silicon or silica cant transport energy that well and life will exsplode. i assume that due to the unknown complications of life and sence mars is the hot or fire planet thay probly dont eat or is self sustained like similar to the marstian manhunter .or is body leaks heat on its own or something to survive. I think when looking at life we need to understand that life can make it and when somethings impossible it makes it possible. All we have to do is make substitutions or imagine around that impossibility .
Reply | Report Abuse | Link to this