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Carbon based life

  1. Aug 24, 2015 #1
    Dear PF Forum,
    I have questions about carbon based life and molecule back bone.
    Is it true that all life on earth are carbon based?
    If it is true, why?
    Is it because carbon is halfway in periodic table? 4 valence?
    What about Silicon, Tin and Germanium? They have 4 valence, why life is not based on them?
    Is it because Carbon the simplest atom then Silicon, Tin and Germanium?
    Why polymer backbone are mostly carbon? Can polymer be constructed from silicon, tin and germanium?
    Perhaps these are too many questions for one thread, my point is all these questions lead to carbon based life. Is carbon based life because of all of those question?
    Thanks for explaning to me.

  2. jcsd
  3. Aug 24, 2015 #2


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    Hit the forum search for "non-carbon life." Read those 187 posts on the subject that have accumulated over the past 10 years.
  4. Aug 24, 2015 #3


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    It is.
    The first life was, and all current life evolved from that.
    There are many arguments why carbon should be ideal, but other options might be possible - we don't know. Those wikipedia articles shouldn't have been hard to find.
  5. Aug 24, 2015 #4
    One problem with Silicon is that it has a strong affinity to Oxygen.
    While Silicon can make long chain molecules as Carbon does, any free Oxygen in the environment would be lethal to a lifeform based on that.
  6. Aug 24, 2015 #5


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    Doh! I just told someone this last week that there are sulphur based lifeforms.
    Thank god it was on Facebook. :biggrin:

    Perhaps I don't understand the term "carbon based". Can it have different meanings?
    If all you eat to survive is sulphur and hydrogen, are you carbon based?


    Or are they just being sloppy in their terminology.
    But the article continues:

    So would it be more correct to say that most life on earth is carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur based, rather than just "carbon based"?
  7. Aug 24, 2015 #6


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    Possibly. I think it depends on what the organism is composed of. Bacteria that use hydrogen sulfide as an energy source still require carbon to form lipids, sugars, and other complex molecules.

    Not that I can see. Carbon is the primary element in almost all complex molecules used by life. Everything from DNA to Proteins to even vitamins are based upon chains of carbon atoms connected together. The other elements you listed are important, but don't form the base from which to build complex molecules.
  8. Aug 24, 2015 #7


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    "NASA announcement: Arsenic-based life form discovered on Earth"
    Yes, plus other "trace elements." H, Li, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Mn, Fe, Co, Cu, Zn, Se, Br, Mo, Cd, I, ... give or take ... have been identified in assays of healthy organisms, or identified as necessary to health of some organisms.
    "Carbon based?" Lipids/fats/oils. proteins/enzymes, carbohydrates/sugars/starch/cellulose, DNA/RNA cell nuclei --- all necessary, and all carbon chemistry. Various S reactions can be observed, both oxidations and reductions, as energy sources to drive the reactions of C based organisms.
  9. Aug 25, 2015 #8
    Ahh, the holder of the 2014 humor award. Okay, let's see how funny it can be!
    Then you'll smell stink! But your body still consists of carbon right. A stinking carbon based I supposed. I think H2S contains energy, and you'll burn it to become CO2 and H2O. The Sulphur? It exits as residue.
    I'll lookup arsenic in Wiki...
    Well, perhaps...
    Frankly, I don't know. Arsenic is in the neighborhood with Carbon group. May be, may be not that life can be based from Arsenic.
    Even if it's been more than 30 years since Junior High School. I still remember my biology teacher, Pak(Mr.) Airbanoe said, CHON for fat and CHONSP for protein. But carbon still the most occurence element in many molecules or compounds or polymer. Life can be formed without six essential elements? I don't know. I"m no chemist for sure, but the probability is very, very small I suppose.
  10. Aug 25, 2015 #9


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    Most organisms need two types of food: one type to provide energy and another type to provide the building material for cellular components (for more information see https://en.wikipedia.org/wiki/Primary_nutritional_groups). For chemoheterotrophs like humans, we derive both our energy and building materials from the foods we eat. However, for plants, these nutrient sources are different: they derive energy from light and building materials from carbon dioxide. For the chemoautotrophic bacteria that the article describes, although they derive their energy from the oxidation of hydrogen sulfide, they still derive their building materials from carbon dioxide.

    It's worth noting that the "Arsenic life" paper has been debunked

    However, the general point that all life uses C,N,O,S,H,P, and various other trace elements is valid.
    Last edited: Aug 25, 2015
  11. Aug 25, 2015 #10
    Of those though, only Carbon has the ability to form complex chains, loops, and other structures, and it easily combines in a nearly infinite number of ways with all the other elements in that group.
    I'd say that's the reason we give the name 'carbon based' to life that we know of.
  12. Aug 25, 2015 #11


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    Speaking of the elements required for life, here's a really nice infographic about the elements found in living organisms:
  13. Aug 25, 2015 #12


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    Well, I saved my best joke for last:

    From the hydrothermal vent life form article I posted:

    Where do they say the bacteria get the carbon from?
    Are there little tiny bottles of "One a day, multiple elements, for bacteria" floating around down there?

    I googled "food without carbon" yesterday, and the only two things people could come up with were salt and water.
    Hardly what I would call "food".

    I also googled "sugar" as I've never taken a biology class in my life, and I did very, very badly in college chemistry, which was 30 years ago, btw.
    Wiki claims that sugars have the general form CnH2nOn.
    Now I know that NaCl is not the only salt in the sea, so maybe there are other forms of sugar. (I really am this stupid.)
    But googling "carbonless sugar" only yielded the obvious: "Carbon free sugar is called water".

    But kidding aside for a moment, I read a very good article yesterday, which was written in very simple language, and I think I may have understood it:

    I thought that was very interesting. If we were silicon based, we would breath in air, and exhale sand! But that only explains why "we" can't be transmutated into silicon based beings. The bacteria either eat or breath, or perhaps both, hydrogen sulphide, and poop out sulphur. hmmmm.... Perhaps that's how we could do it, breath in air, exhale nitrogen, and have sandy poop. hmmm.....

    Anyways, the article kind of concludes:

  14. Aug 25, 2015 #13


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    They get the carbon from dissolved carbon dioxide in the ocean water. There is 50x more carbon dioxide in the oceans than there is in the atmosphere (https://en.wikipedia.org/wiki/Carbon_cycle).
  15. Aug 25, 2015 #14


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    Excellent! You should write up an "Insights" piece on the little buggers.
    Easy enough for me to read though. I look at some most of the titles, and just shake my head.

    Something like the following:
    Is it safe to assume that the nitrogen and phosphorus are condiments?

  16. Aug 25, 2015 #15


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    N, P, and K.
  17. Aug 25, 2015 #16


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    I was supposed to be somewhere an hour ago.
    I'll be back.
  18. Aug 25, 2015 #17
    The term 'food' means nothing, or is really confusing, when we start to talk about autographs. They need to fixate carbon, which is in a low energy state as part of CO2, to form complex molecules to actually build the molecular machines that we call life.
    They need energy to drive processes in directions that are normally thermodynamically unfavourable, as CO2 is a low enegy level and any organic molecule, that can be a machine of life, needs to be lifted up to a higher energy.

    For us humans food is both the carbon source and the energy source, as well as the course of all trace elements and organic molecules we cannot synthesis ourselves.

    Carbon-based chemistry means chemistry where carbon is the backbone of the complex molecules. Sulfur cannot be a backbone of complex molecules, because it cannot form enough bonds.

    Take vitamin b12 or DNA and try to replace the carbon with something else. That is the challenge of finding an alternative to carbon and that is what it means for life to be carbon-based.

    There's many reasons why it is hard to postulate an alternative to carbon. Carbon being abundant, having 4 valence electrons and the energy levels/strength of the bonds it can form, are all important. If carbon-carbon bonds are too weak, the molecules are too reactive. If they are too strong, it will form an inert molecule and not do anything else.
    As it is now, photosynthesis can provide enough energy to break CO2 apart. But one can also put some organic molecules on a table with oxygen around, and it won't spontaneously explode back into CO2.
  19. Aug 26, 2015 #18
    Ygggdrasil has answered that. He answers me, too. See his post below.

    It depends on what the definition of food is, don't you say. Thanks for googled me.

    :oldlaugh:. Is this a science joke? We have C6H12O6 then you remove C, and it becomes 6H2O?

    I think it's a matter of mathematic then. Carbon with 4 valence can easily be combined with other atoms. And from your quote, Silicon is more difficult. The other atoms in Carbon group are Silicion, Germanium, Tin and Lead and they are heavier than Carbon. Perhaps it makes them more difficult to join other atoms than Carbon. I read in Wiki that Germanium is heavier then Iron, and also Tin and Lead.
    So they must have been created in supernova process. Silicon burning is the last stage before iron burning, perhaps that what makes carbon more abundance than Si, Ge, Sn and Pb. Carbon is created in triple-alpha process and in CNO cycle
    That's why carbon is the fourth most abundance element in the universe?
    Abundance Elements.jpg
    But that's according to mass. We have to multiply hydrogen by 16 and helium by 4.
    Luckily Carbon is the fourth most abundant element. And also Hydrogen and Oxygen, they are the most reactive element. Do you think the universe is better without Helium? What good is Helium for life?
    As Brian Green once said, "Our universe is so fine tuned for life"
    It has "just" enough mass to form the galaxy. Not scattered so that galaxy can't be formed. Not too heavy that not long after big bang it collapse again. It's been 13.8 billions years and the universe is still going fine. Many galaxies and we know that our universe can contain life. At least on Earth!.
    So my question is this.
    Why all (if not almost) life on earth is carbon based? Because mathematically carbon is the easiest way to form life?
    Most abundance, easiest reacted, has 4 valence?
  20. Aug 26, 2015 #19
    Good point. Carbon source. I thought food is just energy source.
    I suspect that! But I never read someone wrote that. Now I'm certain that other elements not in Carbon group is more difficult to form bonds because of its valence.
    Very good point. And also to the point, too for my question.
  21. Aug 26, 2015 #20
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