Can silicon be a form of alien life form as carbon is to earth life forms? Elements on group 4 of the periodic table include carbon, silicon, and various other heavy metals. Carbon is the building block of life that we know of. This has gotten scientists thinking about whether it is possible for various other elements to be a basis of extraterrestrial life forms. Most of the analysis for this kind of study comes from Science Fiction, in which they show silicon among other materials to make compounds of various living creatures. Although these concepts are great for reading, they have some flaws in the chemistry behind their thought process.
Carbon and silicon share many characteristics. They have four valence electrons, allowing them to form four chemical bonds from each atom, bond with oxygen, form long chains called polymers, in which they alternate with oxygen. In its simplest form, carbon produces poly-acetal, a plastic used in synthetic fibers and equipment. Silicon produces polymeric silicones, which we generally use to lubricate metal and plastic or even waterproof cloth.
However, when carbon oxidizes with oxygen during combustion, it becomes carbon dioxide, but when silicon oxidizes, it produces a solid, a major factor as to why it cannot support life. The product, silica, also known as sand, is solid because silicon likes oxygen, and silicon dioxide forms a lattice in which one silicon atom is surrounded by four oxygen atoms. These silicate compounds will have SiO4-4 units in them as well. This compound is found in feldspars, micas, zeolites, and talcs, and these systems pose disposal problems for a living organism.
Another factor to take into consideration is that living organisms need some way to collect, store, and use energy, which must come from the environment. Once absorbed, the energy must be released exactly when and where it needs it to be. In other words, if the energy is released all at once, then the life-form will be incinerated. In a carbon-based world, the basic energy-storing molecule is a carbohydrate with the base formula Cx(HOH)y. This carbohydrate oxidizes to H2O and CO2 (water and carbon dioxide), which are then exchanged with the air around them. Through a process called catenation, the carbons are connected into single bonds. Carbon-based life forms are able to use the fuel by breaking it down in controlled steps using speed regulators called enzymes.
These large complex molecules are able to do their job with great precision due to a property known as “handedness.” When an enzyme bonds with compounds it is helping to react, two molecular components fit together perfectly, as a key does to a lock. Many carbon-based lifeforms take this strategy to their advantage. An example can be seen by nature choosing to have the same stable six-carbon carbohydrate to store energy in both human livers and trees. The main difference between glycogen and cellulose is the handedness of a single carbon molecule, which forms when the molecule polymerizes. Cellulose has the most stable form of the two possibilities. However, due to humans lacking the enzyme to break down cellulose into its basic carbohydrate, we cannot utilize it as food. On the other hand, many lower-life forms can break down Cellulose and utilize it as food. Overall, handedness is the characteristic that provides a variety of biomolecules with their ability to recognize and regulate biological processes. The main takeaway from this is that silicon can’t form compounds with handedness. This would make it difficult for silicon-based lifeforms to achieve the proper regulation and recognition functions that carbon-based enzymes do for us.
Despite the many years of research by the likes of Frederic Stanley Kipping, who showed that some interesting concepts can be produced, many silicon analogs of carbon cannot be formed. The highest international prize in the silicon area is called the Kipping award named after the famous Chemist who started the community by thinking about this topic.
It is possible to make a variety of silicon structures that can use solar energy instead of carbon-based carbohydrates, a silicone fluid that transfers oxidants to contract muscle-like fibers, skeletal materials of silicates, silicone membranes, or even cavities in silicate zeolites that have handedness. Some of these structures might even look alive, but the chemistry required to make a living organism is just not there. The complexity 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.
All in all, carbon-based lifeforms are the only known possible form of living organisms and still, as we know it there is no way to create living organisms.
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By: Zubin Sidhu
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References:
“Could Silicon Be the Basis for Alien Life Forms, Just as Carbon Is on Earth?” Scientific American, 23 Feb. 1998, www.scientificamerican.com/article/could-silicon-be-the-basi.
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