What is Life?

Yes.

Is a strand of RNA a form of life by this definition?

Interesting and potentially broad. To me it makes sense to distinguish between collections of chemicals that both promote their own synthesis and do redox chemistry, which I would call "life"; and collections of chemicals that simply replicate, which you define as "life" but I would probably call "proto-life" or "life-like" or something.I think you have to have a metabolism to be alive. Mere self-replication, in my opinion, doesn't allow for the expansive diversity and complexity that characterizes living things. The crucial step, to me, is metabolism.Of course, by my definition battery-powered machines could be alive. The minute Roombas start assembling other Roombas, I guess they're alive.

I guess it would include RNA species, sure. But not prions, before you ask.I think I see where you're going, and I foresee difficulties with my proposed definition. I may have ponder this more carefully.

To be honest I was primarily asking because I don't know the answer to my own questions.I wanted to know where the boundary imposed by your definition lay in terms I could understand.But if it made you think more about the definition in question then I guess that is a good thing.

That sounds like a good way to capture certain concepts of life. I like it.I wonder, though, whether it might exclude certain kinds of "life, but not as we know it". I am not even remotely good at chemistry, but it seems that acid-base reactions (for example) are not redox reactions. Now, can we imagine something which we would like to call "life" having what we would like to call a "metabolism" which use that sort of chemistry instead? Or is there some chemical reason why this is inconceivable? Well, this is exactly why there's a gray area. My gut tells me that viruses should be categorized as life, and in the end our guts are all we have to go on --- there's no objectively correct definition. What if this doesn't involve redox chemistry?

Well I think the problem might be that I haven't shown how to draw a line between the chemicals in question and the "right environment" that I mentioned, and this might become problematic if we pushed it far enough.I could wish my knowledge of chemistry was more extensive.

I don't know if you can "make a living" on acid-base chemistry, because I don't think there's a high enough change in free energy to exploit.For the most part - and I obviously have no way to be sure this is true, just as you have no way to be sure it's not - I think that the biochemistry of life on Earth, particularly of microbes, is so broad and diverse that it essentially encompasses the width and breadth of all the ways it's possible to "make a living" as a chemistry-based organism in this universe. If it's possible to make a living off of acid-base chemistry we should be able to find an organism on Earth that is.Could our mitochondria be an example? ATP synthesis in mitochondria is driven by the energy stored as a substantial pH difference between the matrix and the intermembraneous space. The pH gradient is established by a series of proton pumps that are driven by energy from the TCA cycle (if I'm remembering this right.) So there's a way we're all making a living off of pH changes. But, say, the neutralization of acid by base? It's possible, but maybe we don't see it because the little guys get their lunch eaten by the redox guys who outcompete them. I wouldn't say "inconceivable", but perhaps unlikely. Of course, the one thing that's always true in biology is that you can't say that anything is always true in biology. As full-on life? My gut tells me they're life-like.I don't expect you to ignore your gut for mine, of course, and I'm happy to accept all forms of disagreement on this issue. I'm certain that my definition will eventually be revealed to exclude something I "know" is life and include something I "know" is not. Yours, too. It's the problem with definitions. Well, I mentioned batteries because batteries operate by redox. (All forms of battery. If it's not redox, it's not a battery. Capacitors, for instance, are not batteries.) But, say, nuclear-powered self-replicating robots? I'm prepared to accept that as life even though we're now talking about "organisms" that engage in no chemistry whatsoever. But I think I'd create a different definition for such creatures.

But if they did, you'd want to call them "life", wouldn't you? You see the things I don't know?I do know that if they were battery-powered, the laws of thermodynamics would eventually stop your replication process. You'd want them solar-powered or something.

Can I say "it would depend"?Maybe the definitions game is for mugs, I dunno. Maybe it's just better to go case by case. Sure, ultimately some of them have to be solar-powered, yes.

Are prions life according to that definition?

No. The infectious prions make normal prions change their shape to that of infectious prions. But they do not catalyze the synthesis of infectious prions.

The prevailing theory of prion replication involves "autocatalytic protein misfolding." Why wouldn't that fall within your definition of "life"?Recent research also shows that, although they lack nucleic acids, prions undergo Darwinian evolution, including mutation and natural selection. That finding is irrelevant to your definition, but it would appear to bolster the case for considering prions to be a form of life -- at least in the same sense that viruses could be consider a form of life.Edited by Stephen Push, : No reason given.

I'm in the Viruses are not life camp, myself (and that, as far as I can tell, puts me in the same camp as the majority of Biologists - although perhaps not Virologists themselves). No, because the study of viruses is necessarily linked to the study of the living host of viruses. You cannot understand a virus without also understanding certain aspects of their hosts.

The importance of the differences between life and non-life is the same as the importance of the differences between a rock and a mountain. And where does a rock begin and a mountain end? Is it important to anyone but us?Since we're the only ones using the definition, it's probably best we just make one up; afterall, there will certainly be none discovered within nature herself.If we count it as life, then it is life. How could it be any other way?Jon

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