Silicon vs. Carbon: Why is Silicon-Based Life So Unlikely?

[moe darklight]

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?

 

[DaveC426913]

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?

 

[gravenewworld]

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?

 

[moe darklight]

O i see. so silicon based (or any non carbon-based) life-form is impossible, not just improbable?

 

[DaveC426913]

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?

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.)

 

[DaveC426913]

O i see. so silicon based (or any non carbon-based) life-form is impossible, not just improbable?

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.

 

[Chaos' lil bro Order]

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

 

[DaveC426913]

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

 

[chemisttree]

Yes,

Polysilanes are known. They are unstable in UV light.

 

[Panda]

O i see. so silicon based (or any non carbon-based) life-form is impossible, not just improbable?

Spend a night in Bristol, there are a lot of Trolls their...

 

[gravenewworld]

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.

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.

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.)

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.

 

[moe darklight]

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.

 

[chemisttree]

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).

 

[Moonbear]

Enzymes on Earth are composed of protein (amino acids), sugars and metal complexes, not carbon.

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

 

[chemisttree]

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...

 

[Moonbear]

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...

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?")

 

[gravenewworld]

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.

 

[DaveC426913]

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.

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.

Really how would that be so? Organisms on Earth metabolize carbon based food via enzymes which are extremely large macromolecules.

Some of them do. Think smaller.

 

[moe darklight]

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

hahahaha, never thought of that!

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

 

[chemisttree]

silicon-silicon double bond compounds are disilenes. They have unusual chemistry. Some cyclic ones are even aromatic. http://www.rsc.org/chemistryworld/News/2005/July/19070502.asp

silicon triple bond compounds have been synthesized as well: http://pubs.acs.org/cen/news/8238/8238notw1.html

Carbon may be more abundant in the Universe but it is more important to consider the concentration of elements in your flask rather than in your laboratory.

 

[gravenewworld]

silicon-silicon double bond compounds are disilenes. They have unusual chemistry. Some cyclic ones are even aromatic. http://www.rsc.org/chemistryworld/News/2005/July/19070502.asp

silicon triple bond compounds have been synthesized as well: http://pubs.acs.org/cen/news/8238/8238notw1.html

Carbon may be more abundant in the Universe but it is more important to consider the concentration of elements in your flask rather than in your laboratory.

Interesting articles. However, they even state in them that the silene and silyne compounds are unstable in air and moisture. They are still also very reactive with oxygen, which is even 2x's more abundant than carbon in the universe. These compounds were also synthesized in the lab. That is why chemists exist, to make things that nature doesn't make. It is a huge jump from assuming what a chemist can make in the lab would possibly be able to happen in nature spontaneously. I would like to see examples of silynes and silenes that are found in nature, not synthetically made.

 

[chemisttree]

I also would like to see examples of silynes and silenes in nature. I think Earth is the wrong place to look. They are reactive to molecular oxygen but can you imagine conditions where the element oxygen is present but not reactive with them. The glass vessels the chemists used to make them were composed of silicon and... oxygen.

As for spontaneously forming complex silicon structures in nature... perhaps someone needs to send an electric spark through an atmosphere of volatile silanes or SiF4 and some of the gases like methane, hydrogen and/or ammonia. It would provide a interesting starting point as to what type of chemistry might spontaneously lead to more complicated structures. Yes, I know that SiF4 and silanes react with ammonia without a spark... it is, after all, a huge jump.

 

[GCT]

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?

If you were made of silicon, what would be your mass relative to what it is now? Also, the bond enthalpy of homodinuclear Carbon is significantly higher then that for homodinuclear Silicon. That should be a big clue on a chemical basis for why life essentially exists through a carbon framework basis.

http://www.webelements.com/webelements/elements/text/Ag/enth.html

 

[DaveC426913]

If you were made of silicon, what would be your mass relative to what it is now?

http://www.davesbrain.ca/bio_davesatoms.php" . Carbon has an atomic weight of 12. Silicon is 28. That 16kg becomes 37kg. The silicon man will weigh (21kg more, =) 81kg.

 

[icarus_bill]

Off-subject: silicon-based foods

Hi there. I'm new to this forum, so I hope to find some fun, creative thinkers, as well as some hard-nosed curmudgeons.

I'd like to put forth a hypathetical question:

If a being subsisted on silicon-based foods, what would he eat in our world; what silicon-based products would be comparable to our normal food?

-Bill

 

[Panda]

Hi Bill, Welcome to PF

The answer is Simple - Assuming the silicon or any other element base life form evolved on this planet, they would have evolved from simple organisms processing minerals through to a series of more complex silicon based life forms let's call them splants feeding off the simple lifeforms into highly complex life forms or animals feeding of splants and other animals. If there were no simple carbon based life forms we could not have evolved.
If however the silicon based lifeform arrived here from the stars, then they would starve.

 

[icarus_bill]

Hi Panda. Thanks for replying.

Let's see if I got this straight, or if I'm forcing an idea. I'll start here:

A carbon-based life form could evolve to subsist on silicon-based energy sources (foods) through its evolution environment?

 

[Chaos' lil bro Order]

Hi Panda. Thanks for replying.

Let's see if I got this straight, or if I'm forcing an idea. I'll start here:

A carbon-based life form could evolve to subsist on silicon-based energy sources (foods) through its evolution environment?

I don't think your question can be answered. Its too speculative and there is no scientific basis for pursuing this inquiry in either direction.

 

[DaveC426913]

A carbon-based life form could evolve to subsist on silicon-based energy sources (foods) through its evolution environment?

What? No. Read his post again.

He's saying that a silicon-based higher-lifeform would not have evolved in a biological 'vacuum'. It would have evolved in an entire ecosystem where there is a complete silicon-based food chain beneath it.

As an analogy, we (being carbon-based lifeforms) did not get to be highly-evolved by having to make our own food. We eat carbon-based critters (by they plants or animals) have have done all the hard work for us.

Likewise, a silicon-based higher lifeform will rely on lower silicon-based lifeforms to concentrate its chemical energy sources.

 

[icarus_bill]

Please let me know if my queries are inappropriate for this forum, and if so, any alternative forum suggestions would be greatly appreciated.

I realize I'm pushing an unnatural idea, but it's for a story I'm writing. So, without getting into too much detail at this point, let's use those magical words, "What if..."

What If: a man from a parallel dimension, who eats silicones (gels, etc.) crosses over to our dimension?
- What could he eat here, in our dimension, off the shelf? (hair-care products, cosmedics, etc.)
- Are there different "grades" or "kinds" of silicones that he would be prone or adverse to?

What If: silicone gel is required for the trans-dimensional trip?
- could it behave as both an energy conductor and a buffer for the traveler?

Again, this is a fun, creative inquiry, not so much a science fact check.

 

[DaveC426913]

This forum

http://www.bautforum.com/

is a little more lenient on wild speculation.

 

[Chaos' lil bro Order]

Please let me know if my queries are inappropriate for this forum, and if so, any alternative forum suggestions would be greatly appreciated.

I realize I'm pushing an unnatural idea, but it's for a story I'm writing. So, without getting into too much detail at this point, let's use those magical words, "What if..."

What If: a man from a parallel dimension, who eats silicones (gels, etc.) crosses over to our dimension?
- What could he eat here, in our dimension, off the shelf? (hair-care products, cosmedics, etc.)
- Are there different "grades" or "kinds" of silicones that he would be prone or adverse to?

What If: silicone gel is required for the trans-dimensional trip?
- could it behave as both an energy conductor and a buffer for the traveler?

Again, this is a fun, creative inquiry, not so much a science fact check.

Bill. Since you are writing fiction, you can just make up what you like. There are few rules of thumb in chemistry as to what is poisonous and what is not. If you want a rough guideline I can give you some tips as to what molecules tend to be detrimental to human health.

Dioxins (molecules built around two singly bonded oxygen atoms)

Most substances that contain halides like CL, BR, F, etc. (this is because these atoms have such a high electronegativity that they tend to break apart other normally stable biological molecules). Popular examples include Chloroform, Fluorides, Hydrogen Bromide, etc.

Alkaloids, which is a fancy name for bases, tend to be detrimental to humans since our body is acidic by nature and much perfers them. Of course there are many healthy foods that are alkaloid. But some popular toxic alkaloids include Ammonias, Indoles and amines (for amines, think of methamphetamines, morphines, heroin(es), etc.) All of these alkaloids mentioned are nitrogen-based, but they don't have to be.

Undigestables like celluloses found int tree-stuffs. The reason humans cannot digest things like wild grass and tree leaves is because they are primarily cellulose based. We lack the proper enzymes (keys) to open up and unlock the energy in these molecules, therefore they simply pass right through our digestive tracts. Cows and grazing animals on the other hand have the proper enzymes. Interestingly, cellulose is nearly identical to many sugars like glucose, except for a tiny area of the molecule where an -OH is affixed instead of an -H. Enzymes are very picky and just like keys, you know that one extra notch means the lock just ain't going to open!


I have tried my best to help you. In my opinion it is quite likely that a silicon-based 'human' would encounter the same or similar problems metabolizing such toxic molecules as we would. But as many people have pointed out this thread, its really impossible to know this so we can only make semi-educated guesses.

Hope you find this usefull. Chao.

 

[Roy Edmundson]

Lets look at the situation with carbon based life as we know it. There are complicated molecules for storing genetic information (DNA), mechanisms to translate this information into structural molecules, enzymes and hormones(proteins) and biochemical pathways to store produce and use chemical energy - carbohydrates and fats.
While silicon chemistry is similar to carbon there just isn't the same wide variety of compounds avaliable that would be able to do all these jobs. Large molecular weight silicon compounds exist, but are nowhere as structurally rich as DNA or protein. mostly they are just silicon and hydrogen without nitrogen phosphorous or oxygen. So its dificult to see how silicon life could store and use the necessary genetic information.
Carbon life is very compatible with water as a solvent . A solvent alows the movement by brownian motion of all the biochemical compounds within the living cell - a viable silicon lifeform would also need mobility, however water is out and there doesn't seem to be any other alternative solvent . Carbon life with water as a solvent is able to handle most simple carbon compounds without geting wrecked - ie poisoned. carbohydrate metabolism has pathways to utilise a great many of the small molecules containing carbon - eg acetate, malate succinate, ethanol, glycerol, acetone. there are relatively few poisons that life often encounters - most of us die of old age rather than being poisoned. Silicon life looks likely to have many more problems in this department - oxygen might be an unsurmountable problem because SiO2 is a solid. Water might be a poison as well.

I still believe in the possibility of silicon life, but I think that it might well be electronic rather than chemical.

 

[Jimmy Snyder]

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

Much as our diamonds and pencils are made of meat.

 

[DaveC426913]

Much as our diamonds and pencils are made of meat.

Good point!

 

[DTSchaefer]

Much as our diamonds and pencils are made of meat.

well, that's a good point, because the molecules that make up living organisms are far more complex than the crystalline carbon structures of diamond/graphite/nanotubes/etc. Who is to say that silicon-based life isn't made of the same large molecules?

So, same substance, different form, NOT MEAT. For both S and C based life.

 

[vaazu]

But if we would assume that the pressure and temperature wouldn´t be the same in the other enviroment. Then could Si compounds be more stable? And the oxidizer (i hope it´s like that in english) wouldn´t be O. For example F. Then SiF4 (it should be a gas) could play the role as CO2. Or is F so strong oxidizer that it would oxidize any Si compound, so the life could not stay? Or maybe the life could live in a liquid environment where the liquid is reductant, so the energy would be generated by reducting food. How unlikely is reducting atmosphere? Would H2 be it?

Hope anyone can catch my ideas :D and say anything about them.