Carbon-based lifeforms?

We’ve all heard the statistics that anywhere from 2/3 to 3/4 of the human body is composed of water (Wikipedia says 72%..)
If the we are made mostly of water, why are we always described as “carbon-based lifeforms”? Is it just assumed that any lifeform will contain a lot of water so they would be classified by the second most common compound?

*Mods: This could have gone in CS, since the distinction between carbon-based and non-carbon-based shows up mainly in science fiction, but I’m looking for a factual answer (if one exists.) If not, feel free to move this thread.

The water acts a solvent for the carbon.

Yeah; carbon is the thing that does all the interesting chemical interaction; water is just the solvent.

I would not refer to water as a “solvent” for carbon, since many carbon compounds are not very soluble in water. Water does, however, serve as a medium in which many biochemical reactions can take place.

Carbon is unique among elements in the number of compounds it can form, since it is the smallest atom that can form bonds with up to four other atoms. This enables it to form the backbone of a huge variety of very complex compounds; such complex compounds are essential for life.

Silicon can also form four bonds, and so it is theoretically possible to imagine silicon-based life forms. However, there are certain chemical limitations that make silicon-based life less likely than carbon-based life.

Carbon compounds do all the interesting stuff. It used to be that they thought “organic” substances couldn’t be synthesized by non-biological process, until the late 18th/early 19th centuries. Most of your important life chemicals are made of Carbon, Hydrogen, Oxygen, and Nitrogen, with the occasional odd other atom – Sulfur, Iron, whatever – thrown in. Carbon has the ability to form chains and circles and benzene rings that give it a unique capability to act as the basis for an entire realm of chemistry.

Every now and then someone suggests another basis for life. Typically it’s Silicon, which can also bond in a variety of ways, and lies beneath carbon on the periodic table. But I don’t think Silicon has anywhere near the versatility of carbon, and there are other problems (Silicon dioxide, the analogue to carbon dioxide, is basically sand, and woulod make a miserable waste product)Or someone will note that Chjlorine gas will support a flame, and think about chlorine-based biologies. I’ve never seen these analogies pushed past these simple observationsinto full=blown biological systems, though (Although I attended a lecture Hal Clement gave on “BleachWorld” at a SF convention about five years ago.)

And what did Wöhler exclaim when he first synthesized an organic compound from inorganic constituents?


Thanks. I’;d forgotten who synthesized it, and what it was.
Only I’m not sure finding out was worth the price.

I think Frank Edwards, in one of his “Strange” books recounted the story of someone who actually synthesized silicon-based life forms resembling toads. I’ll have to see who the “scientist” was when I can put my hands on them.

Yeah, he did. I’ve read about it in other books, as well. It’s based on a self-publiashed book by the guy who claimed to do it. He mixed up chemicals and drpped a big chunk of iron oxide in the middle of it, then claimed that winge insects grew from the ball. Right.

As a kid I tried duplicating the experiment, with negative results. It’s more than a little hard to believe that anything as complex as insects could grow from so simple a mixture of inorganic chemicals. I can’t even believe that the gfuy was simply mistaken, and mosquitos,. say, had laid eggs in his experiment. The mixture was basically a toxic waste dump in a jar, and I can’t see mosquito larvae growing in it.

As for chlorine, that wouldn’t replace carbon, it would replace oxygen. It would be pretty easy to imagine an ecology where plants used photosynthesis to create some sort of analogue of sugar and released chlorine gas into the atmosphere instead of oxygen. And animals would then breathe chlorine and excrete carbon tetrachloride or some such instead of CO2. Chemically chlorine or florine could easily take the place of oxygen as an electronegative compound.

But the trouble is that oxygen is much more common than chlorine or florine in the universe. There’s lots and lots of hydrogen, then helium (which is inert and so irrelevant), then carbon, nitrogen and oxygen. And this is due to the peculiarities of how nuclear fusion works, some elements are created preferentially to other elements.

So if you compare the amount of chlorine on earth to the amount of oxygen there’s really no comparison. There would have to be some mechanism to preferentially enrich a world with chlorine if you want to imagine a science fiction style chlorine breathing alien.