What makes life alive?

24 02 2010

Pebble plants (Lithops salicola) among rocks. What distinguishes one from the other?

What is it, exactly, that makes a thing alive? Why do we call a rabbit alive, but not a rock? Here’s a little dialog to help clarify the problem. This is hardly comprehensive and I reserve the right to make sweeping edits in the future! But here it is, just below the fold:

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Q: I’ve been wondering. What exactly is the difference between living things and nonliving things?

R: Usually this question is answered by listing a number of qualities which must be possessed for a thing to be considered alive.

Q: Like what?

R: Qualities like order, evolutionary adaptation, response to environment, self-regulation, energy processing, reproduction, and growth and development.

Q: Hm. But I can think of something which satisfies just about all of these, yet we don’t call it alive! The virus!

R: Right, we don’t call them “alive.”

Q: But why not? They reproduce, they evolve and adapt… They sure seem like living things to me. At least, they’re a heck of a lot more alive than a rock is!

R: You’re on to something here! It does seem strange that we would group viruses with rocks instead of, say, bacteria. In fact, what you’re revealing with your question about viruses is that our definitions of “living” and “non-living” matter are strictly human categories into which nature need not fit itself! The categories are far from entirely arbitrary or useless, but they are human inventions nonetheless!

Q: So you’re saying these fuzzy cases like viruses are only tricky because our categories are conceptual constructs overlaid on reality?

R: Exactly.

Q: But you did say these categories are useful.

R: I did. They certainly help organize our conceptions of the world. Breaking reality down into simpler pieces or groups (like living/nonliving) helps us to think about them and make predictions. So for instance, we classify both a frog and a tree as living things. Simply by acknowledging that both the frog and the tree belong to the same category, we can assume some things about the tree just by looking at the frog. For instance, we see the frog is made of cells, reproduces, needs energy to function, and so on. A tree has these qualities as well, and we can assume that just from their being in the same category. So the traditional definition of life is far from useless.

Much unites this Green Leopard Frog with the small plants on the surface of the water.

Q: I see…

R: And, there certainly is matter which is in many ways very different from that contained in, say, a rock.

Q: How is “living” matter different, then?

R: It’s different in a number of ways. For instance, let’s talk about it from a chemical perspective.

Q: A “chemical perspective?”

R: In terms of the nature of the molecules, the rates and variety of reactions, and so on.

Q: I see… OK, go ahead.

R: So let’s compare two things which anyone would agree are very firmly in their respective corners of the living/nonliving divide. Representing the living is a rabbit. Representing the nonliving is a common rock.

Q: Rabbit – living. Rock – nonliving. Got it.

R: Good. So the chemical behavior of these two things are VERY different. For instance, inside the rabbit, a huge variety of chemical reactions occur, including those involved in digestion, synthesis of new parts, DNA replication, and so on. However, the reactions of the rock are mostly limited to surface atoms dissolving in passing water, or being stripped away by wind or abrasion with other hard objects. The interior of the rock can react, but only under conditions of high temperature and pressure.*

Q: So what it comes down to is the variety of reactions? That’s what really separates living from nonliving?

R: Not just the variety of reactions, but also their rate.

Q: You mean how fast they go.

R: Correct. While the surface reactions involving the atoms of the rock are generally rather slow, the reactions in the rabbit proceed extremely quickly. This is because the rabbit contains enzymes – large protein or nucleic acid molecules which catalyze (or speed up) reactions by lowering the amount of energy needed for these reactions to proceed. So the rabbit and the rock differ not only in the number of different types of reaction which occur, but also in how fast their respective reactions go.

Q: Makes sense. But I have something that contradicts what you’ve just said. The organisms of the phylum Tardigrada!

R: I had a feeling you were going to bring those up…

Q: Well yes! Because the tardigrades are capable of almost totally suspending their chemical reactions and entering a state of near total inactivity! So when they’re like this, they aren’t that much different from a rock in terms of rate of reaction!

Modern tardigrades. They're very small, and exist in a huge variety of environments. They are known as polyextremophiles, because they can withstand numerous conditions that would kill most living things. These include dehydration, extreme heat, extreme cold, extreme pressure (high and low), and extreme radiation. There are probably some in your house right now!

R: Excellent point! So the differences can’t be reduced to differences in chemical behavior.

Q: That’s what it seems to imply to me.

R: Yes, you are right. And there’s a larger point we can make while we’re here. An aside, before we move on.

Q: What’s that?

R: That the reactions of the rock and the reactions of the rabbit are actually both a part of a much larger and more intricate chemical system!

Q: Really? How so?

R: Here’s an example. Say our rock falls into a river. Over time, the rushing water knocks atoms and molecules off of the surface of the rock and they enter the water flow as dissolved ions.

Q: You already mentioned this…

R: I know! Be patient! So the rock’s molecules are now in the water as dissolved ions. Well what if these ions happen to bump into the root of a nice leafy green plant?

Q: The plant will take up the ions and use it!

R: Yes…

Q: … Making these ions a part of a living system…

R: Yes…

Q: And demonstrating that the distinction between living and nonliving cannot be applied to a given atom in itself, but only in the context of the chemical system the atom is currently participating in! Furthermore, it reveals that everything on Earth from a rock to a plant to a rabbit participates in a single chemical cycle far larger and more complex than any individual component of the system!

R: Well said!

Q: It’s like we’re two aspects of the same being.

R: Indeed! So, I trust you are satisfied now?

Q: Absolutely not!

R: No?!

Q: No!! You never resolved the Tardigrada conundrum!

R: You’re right. You know, it seems like you’re looking for some special characteristic of living matter that lets it do all these amazing things like coming back from a decade of inactivity as if nothing had ever happened – some property or quality that the matter in the rock doesn’t have.

Q: Yes! That’s exactly what I’m after!

R: Well, I think there is something that will satisfy you. Structure!

Q: Structure?

R: “The beauty of a living thing is not the atoms that go into it, but the way those atoms are put together.” – In other words, their structure!

Q: I don’t follow.

R: So in the rock, the atoms are arranged in a sort of crystal lattice. It’s more complicated than you might expect, but it’s fairly uniform. Now, in a living thing, the structure is MUCH more complex.

This is calcite, a mineral found in limestone.

Q: Why? How so?

R: Well first let me give a caveat and reiterate that the matter in a rock is the same sort of stuff as the matter in the rabbit. But the matter in the rabbit is arranged in a much more intricate manner. First of all, it is based on carbon.

Q: Carbon?

R: Yes! Our friend, element #6. Carbon’s electron configuration allows it to form long chains to which other atoms or molecules can bind. This allows for a huge variety of different compounds, each of which has its own particular chemical behavior. With the huge diversity of molecules that can be created from a carbon framework, a reaction involving carbon-based matter (and with constant energy input as we get from the sun) can develop to be far, far more complex than a reaction only involving simpler molecules.**

This is maitotoxin, a carbon-based poison. Each point where two or more lines come together represents a carbon atom. See how large and extensive a carbon-based molecule can get!

Q: Why, exactly?

R: Part of it has to do with size. Let me put the difference in size in perspective – a molecule like water, that’s got a molecular weight of about 18 atomic mass units. Not bad, but organic molecules can get huge. Really huge. How huge? Consider hemoglobin, with a molecular weight of 68,000 atomic mass units. Or the colossal protein called connectin, weighing in at 2,993,442 atomic mass units.

Basically, organic molecules can be REALLY REALLY BIG. This huge size allows a tremendous diversity of structures, and this allows a tremendous diversity of chemical reactivities. Such complexity allows structures which remind us greatly of our own mechanical contraptions. And this in turn lets them do complicated things like catalyze chemical reactions.

My friend and yours, the protein ATP Synthase. Without this fella you wouldn't be doing much of anything, because it produces the "energy currency" of the cell - ATP. It's also carbon based and shows the incredible complexity possible in organic chemistry. It should remind you of a rotor.

Q: I think I’m seeing it now… The major difference between living and nonliving matter is the structure of the molecules involved! So the Tardigrade in suspended animation is different from the rock because even when the creature’s metabolism slows WAY down, the molecular machinery is still there inside the animal in good form, ready to resume the reaction at full speed when the solvent becomes available.

R: That’s it.

Q: Wow, we’ve covered a lot of ground. Let me try to summarize. Living/nonliving is a human dichotomy which only partly reflects the real divisions among matter. These real divisions include variations in number of reaction types, the rates of these reactions, and the structure of the involved molecules. However, the traditional definition of life, which involves traits like growth and development, order, energy processing, and so on, also has its place as a way of conceptually organizing the world.

R: You got it!

Q: BOOYAH!

Questions? Concerns? Complaints? Corrections? The comments section awaits your input!

-Neil

*We should also note that in inorganic liquids and gasses, the full volume, not just the surface, is available for reaction.

**Silicon has a similar chemical behavior, but on Earth it seems to be mostly tied up in rocks. Still we can’t preclude the possibility of silicon based life elsewhere!

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