Silicon vs. carbon

  1. My understanding is that it would be very unlikely (if not impossible) for a non-carbon based life form to exist because of carbon's incredible ability to bond (it can both get or give up 4 electrons). But silicon also shares this same property.
    why is organic material so much more common, and why is silicon-based life unlikely? shouldn't both these elements be just as good at forming bonds in nature?
     
  2. jcsd
  3. DaveC426913

    DaveC426913 16,365
    Gold Member

    My science teacher (who carved his explanation into his stone tablet) said that silicon molecules would just fall apart. It doesn't have he holding strength of carbon.

    Actually, it should be simpler to figure out: do we see much in the way of complex silicon molecules? It doesn't form chains does it?
     
  4. carbon is more abundant than silicon. if an organism were to perform metabolism that would involve oxygen, how would it get rid of silicon dioxide (the counter part to CO2) since, as we all know, it is a solid?
     
  5. O i see. so silicon based (or any non carbon-based) life-form is impossible, not just improbable?
     
  6. DaveC426913

    DaveC426913 16,365
    Gold Member

    I don't see that as any kind of showstopper. What's the big deal?
    1] There are critters here on Earth that expel solid waste.
    2] You're making some big assumptions about how it would be built. It may not need to expel that much silicon. It's really about how the organism extracts energy from the molecules it processes, and then what products count as waste. There's no reason to assume these silicon-based critters don't still take in carbon. That way, they could still get rid of waste by combining the carbon and the oxygen (now that all the useful energy has been extracted from them in their initial bonds.)
     
    Last edited: Feb 14, 2007
  7. DaveC426913

    DaveC426913 16,365
    Gold Member

    Improbable.

    No one can say any form of life is "impossible". But it would definitely fall outside the category of "life as we know it", and thus is really just wild speculation.
     
  8. I think the central idea to emphasize is that silicon bond lengths are much longer than carbon bond lengths. This means, as Dave pointed out, that silicon bonds are weaker than carbon bonds. Therefore, structurally, a silicon backboned molecule is weaker and also less likely to form than a carbon backboned molecule. IMO its much more likely to find carbon based aliens than than silicon ones. A second point that supports this is that the abundance of carbon in the Universe is much greater than that of silicon. This is due stellar evolution, see here:http://observe.arc.nasa.gov/nasa/space/stellardeath/stellardeath_1c.html
     
    Last edited: Feb 14, 2007
  9. DaveC426913

    DaveC426913 16,365
    Gold Member

    Are there some Earth-based examples of long-chained silicon molecules?
     
  10. chemisttree

    chemisttree 3,721
    Science Advisor
    Homework Helper
    Gold Member

    Yes,

    Polysilanes are known. They are unstable in UV light.
     
  11. Spend a night in Bristol, there are a lot of Trolls their... :devil:
     
  12. I would like to know of some critters that create solid waste via cellular respiration. Expelling sand would be much more difficult for an organism than expelling CO2. It is no wonder why evolution chose carbon based life forms here on Earth. Remember we are talking about waste products from respiration, not digestion.



    Really how would that be so??? Organisms on earth metabolize carbon based food via enzymes which are extremely large macromolecules. You said so yourself that silicon could not form long chains, so how could a silicon analog of an enzyme exist? No enzymes=no/slow metabolism of things like glucose. If an organism isn't made of carbon, I don't see how it could reasonably create enzymes made of carbon.
     
  13. completely hypothetical obviously:

    why would it have to breathe Oxygen? maybe it lives in a planet that is not earth-like at all. ... maybe it doesn't even breathe in the same sense we do, considering it would be so different to begin with.
     
  14. chemisttree

    chemisttree 3,721
    Science Advisor
    Homework Helper
    Gold Member

    Silica need not be expelled as a solid. Silica is sort of soluble in water, for example. Think of the diatomites or sugar cane or rice. The ash of these once-living creatures contains a fair amount of silica in the form of diatomaceous earth and bagasse ash or rice hull ash. 27.7% of the earth's crust is composed of silicon. Carbon is less than 2%. Carbon is not more abundant than silicon. Enzymes on earth are composed of protein (amino acids), sugars and metal complexes, not carbon. Long chain carbon molecules are not requisite for life as far as I know. Long chain carbon molecules that contain hydrogen and a polar end group are the norm for us (fat and lipids). Do they have to exist for life though? So far we haven't found anything different on Earth. Phosphosilanes might fit that niche.


    What a strange world it would be! It might be devoid of oxygen and strongly reducing conditions might be the norm. Or if oxygen were present, the environment might be highly acidic or strongly basic, facilitating the transfer of silicon around the "body". Fluoride/fluorine might be the redox couple that replaces the oxygen/water redox couple. Energy might be derived from UV rather than from chemical sources. How would energy be stored? Probably not as carbohydrate! How about polysilane? Polysilazane? What would the enzymes look like? Probably not like ours but they would have to be flexible, have specific shapes and reversible binding. Chemical changes to their structures would have to affect the binding affinity and change the presentation of active sites. What kind of active sites would these putative enzymes have? Solution-based processes are the norm for us. Water is the solvent for us. What type of solvent would these creatures use? Liquid methane or ammonia or CO2?

    But strangest of all.... their telescope optics would be made of meat! (their version of meat, SiO2) Or perhaps dressed up corpses (coated optics).
     
  15. Moonbear

    Moonbear 12,265
    Staff Emeritus
    Science Advisor
    Gold Member

    :confused: Proteins and sugars both have carbon backbones. Why would you say they don't have carbon?
     
  16. chemisttree

    chemisttree 3,721
    Science Advisor
    Homework Helper
    Gold Member

    Protein has a polyamide backbone. Nitrogen is interspersed every two carbons. Sugars and oligosaccharides are hung on it like shrubbery (shrubbery?). They are there for solubility and to tailor the quaternary structure. Polysaccharides have 6 carbons alternating with an oxygen.

    I didn't say that proteins didn't have carbon, I said that they weren't composed of carbon (referring to a previous comment "made of carbon"). A slight nuance on the meaning.

    The point was that the properties of carbon (graphite, carbon dioxide, diamond, buckminsterfullerene) don't really fully describe the potential of a compound containing other species like oxygen, nitrogen, phosphorous, halide, etc...
     
  17. Moonbear

    Moonbear 12,265
    Staff Emeritus
    Science Advisor
    Gold Member

    Okay, you were referring to having properties of more pure carbon compounds, not whether the molecules had carbon atoms in it. Just wanted to clarify that. (And now you're going to have Monty Python stuck in my head all night..."Shrubbery?")
     
  18. and how are enzymes made? through DNA, which is composed of a base which contains carbon and a sugar/phosphate backbone which contains more carbon.

    if i remember correctly, carbon is definitely much more abundant in the universe than silicon. maybe not in the earth's crust, but in the universe yes.

    silicon is also not known to make double and triple bonds with another silicon atom. this would also be a huge drawback of trying to base life on silicon.

    energy from a UV source? i thought you posted earlier that polysilanes aren't stable in UV radiation... If a silicon based organism were to use UV radition for energy and were to need an environment like liquid ammonia, methane, co2 how much UV radiation could it even get?? the boiling points of those liquids is extremely low, i doubt that much energy in the form of UV radiation would be able to reach those organisms since the liquid methane etc. would just evaporate off.
     
  19. DaveC426913

    DaveC426913 16,365
    Gold Member

    I still think you're making some big assumptions. Bacteria shouldn't have too much trouble expelling waste products. Are you thinking of large organisms?

    In terms of solid explusion, I was actually thinking of sea lizards that excrete waste salt from their eye ducts.



    Some of them do. Think smaller.
     
  20. hahahaha, never thought of that!

    p.s: so would their computers, creating a really grey are between living and A.I, eh.
     
  21. chemisttree

    chemisttree 3,721
    Science Advisor
    Homework Helper
    Gold Member

Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?