March 15, 2004

WHO'S THE LEADER OF THE GANG?:

Seeking Life as We Know It: To all appearances, it has to start with water -- but does it? What is the likelihood of an ammonia-based alien somewhere in space? (K.C. Cole, March 5, 2004, LA Times)

On the face of it, water seems a rather silly molecule — two hydrogen atoms attached to an oxygen atom in a way that looks like the head of Mickey Mouse. Even children know its chemical formula: H2O.

But the bonds it forms with itself and other molecules are anything but ordinary.

Atoms normally bond by sharing the negatively charged electrons that buzz around their positively charged nuclei, like people sharing popcorn at a movie.

In water, the oxygen shares one electron with each of its hydrogens, leaving four extras. These clump together as "lone pairs" that can grab onto other molecules like prehensile feet.

At the same time, the two positive hydrogen nuclei stick out the other side like arms. The "feet" of one water molecule grab the "arms" of the other, forming abnormally strong networks. Where one water molecule goes, the others tend to follow. Thus, water can climb tall trees — hand over foot, as it were — in defiance of gravity, carrying nutrients from the soil to the leaves.

Chemists say they would expect water to be a gas at room temperature because it's made up of just a few light atoms. But the strong bonds make the molecules stick together in a liquid form.

Luckily, the bonds aren't so sticky that they form a viscous gel — something that Boston University physicist Eugene Stanley initially found perplexing. Water flows freely, he and others discovered, because water molecules stick to each other only briefly, let go, grab another partner — whirling an ever-changing cast of partners around in a molecular square dance.

The upshot is that water stays watery over a remarkable range of temperatures (32 to 212 degrees Fahrenheit, to be exact).

This is a liquid bonanza for life, which seems to need some form of fluid to transport things from place to place. In solids, molecules stick together and can't go much of anywhere. In gases, the molecules don't get close enough to interact.

Water's unbalanced geometry — positive charges on one side, negative on the other — also gives it a distinctively schizophrenic personality (although chemists, like psychiatrists, prefer the term bipolar). This makes it an excellent solvent.

One side of a molecule grabs on to negative charges; the other side grabs the positive. This pulls most things apart, so water can dissolve almost anything. (If things didn't dissolve, they'd sink to the bottom, or rise to the top — not good for a free flow of chemical reactions.)

Why doesn't life just disintegrate altogether in water then? While water is one of the most strongly bipolar molecules, it is not the most reactive — meaning it can make things fall apart (dissolve) without changing their composition (react). So the parts can be endlessly rearranged.

And as it turns out, the few things water doesn't dissolve are equally important in assembling life's building blocks. Water hates fat. "It won't dissolve a spot of grease on my nice silk tie," Stanley said.

Water herds these hydrophobic (water-hating) and hydrophilic (water-loving) molecules into structures such as cells. The hydrophobes point away from each other, while the hydrophiles look inward. "It's like circling the wagons," McKay said.

Water, in other words, gives living things outsides and insides. The hostile outside is kept at bay, while inside, the proteins behind nearly all of life's mechanisms go about their business.

"You have 3,000 proteins, minimally, in every cell," said University of Massachusetts biologist Lynn Margulis, "and every reaction requires water. Everything else is negotiable."

What's the water doing with the proteins exactly? "Everything," Margulis said. "It's like a loom that you can do the weaving in. It's the matrix that's holding things in place. Nothing can go on without it."

The magical molecule does a whole lot more: For example, it absorbs heat slowly, and holds on to it for a long time. This stabilizes temperatures not only in the oceans, but also inside living things — which, lest we forget, are made mainly of water.

Finally, water expands when it freezes, contrary to nearly every other substance known. That's why ice floats, allowing it to form an insulating blanket on lakes and ponds for life beneath. Without it, fish would freeze before they hit the grocer's shelves.

Of course, it's hard to ignore one obvious reason life may depend on water. Hydrogen is the most abundant element in the universe. Helium is the second, but it's inert — so standoffish it doesn't bond with other atoms at all. Oxygen comes third. Maybe life is made of water simply because it's there.

But some otherwise habitable worlds just don't have water. Are they out of luck?

Not necessarily. "Water's a wonderful molecule," McKay said, "but there are other wonderful molecules."


Are molecules really perfectly round and do the H's and the O really line up in a V?

Posted by Orrin Judd at March 15, 2004 5:18 PM
Comments

(a) No and (b) Yes, but an open V - about a 120 degree angle if I recall correctly.

Posted by: pj at March 15, 2004 5:26 PM

And they stay in one spot? And we can observe them?

Posted by: oj at March 15, 2004 5:31 PM

Wait, wait a sec. The bond that you see as a V is actually just a physical description of a mathematical probability. In the same way that an electron "cloud" is simply all of the points at which electrons could reside, so is the bond simply a convenient shorthand for where electrons are hanging out, and the "shape" of the molecule at an average moment.

OJ: We can observe atoms and molecules, in the sense to which you refer. But the much-misunderstood uncertainty principle keeps us from "observing" bonds, let alone individual electrons.

Posted by: Chris at March 15, 2004 5:40 PM

We can calculate the bonds because we can measure the weak force, to a fair degree of accuracy -- about one part in 10 billion. Only certain relationships are allowable.

Posted by: Harry Eagar at March 15, 2004 9:56 PM

"Only certain relationships are allowable"

And, what does the ACLU think about that?

Posted by: Uncle Bill at March 16, 2004 11:03 AM

Amazing stuff, that "dihydrogen monoxide" that fools the activists into trying to ban it every few years...

And I have a Poul Anderson non-fiction book from the early Seventies (found in the Occult section of a used bookstore -- they must have looked at the title Life on Other Worlds and thought it was about UFOs) that speculates about Ammonia as a solvent for not-as-we-know-it life. The idea's been kicking around for a long time.

Posted by: Ken at March 16, 2004 12:24 PM

Mr. Eager;

No, the weak force doesn't have any effect on the shape of a water molecule. The shape is determined by the electron orbitals. And while the electrons are pretty fuzzy, the nuclei are relatively stable and in fact all lie in the same plane with a roughly 105° angle between them. You also need to not think of the standard "ball on a stick" model, which is useful for modeling but not that close to physical reality. Because the effective molecular shape is determined by the electron clouds, there's no "edge", in the same way an atmosphere doesn't have an "edge". It just fades out and the edge is more of a convention than a physical reality.

Further, according to what I found while looking for a picture, there's still some dispute about how exactly a water molecule works and what the O-H angle is (it seems depends on the state of the water in aggregate, i.e. it's different for liquid vs. solid).

Mr. Judd;

I'm surprised that you didn't bring up the fact that the properties of water are a frequent Intelligent Design argument. It's such an odd substance, yet all of its oddities are ones that favor carbon based life. I find this one of the most compellling ID arguments, probably because it's exactly the kind of thing I do in my own designs. Rather than heavy handedly forcing a result, one tweaks the fundamental strutures / rules so that the desired outcome is inevitable.

Posted by: Annoying Old Guy at March 16, 2004 2:48 PM

AOG:

The entire story is an argument from design, isn't it?

Posted by: oj at March 16, 2004 3:06 PM

As I understand it (rather fuzzily, perhaps, like an electron moping about its orbit), it's the weak force that establishes the orbits. The orbits establish the bonding moments.

Posted by: Harry Eagar at March 16, 2004 7:56 PM

Another fascinating property of water is that it becomes less dense when it freezes, thereby allowing the ice to form on the surface of a lake, river, etc. Most liquids do not share this. If water were different, more than just our drinks would be affected.

Posted by: jim hamlen at March 17, 2004 2:00 PM

Yes, we can all admire water, our versatile friend.

I suspect that one reason we are not 98% ammonia is that nitrogen is much less reactive than oxygen, and any planet violent enough to create lots of ammonia is likely to be too rough for life.

Posted by: Harry Eagar at March 18, 2004 1:20 PM

Provided that you assume life is intended to be like it is here

Posted by: oj at March 18, 2004 1:26 PM

There's a question in my mind, purely speculative, about whether natural selection would break down if the pace were greatly accelerated from what we're accustomed to.

As far as I know, no darwinian theoreticians have ever addressed this, but it would seem likely that while great amounts of time are needed for speciation, merely large amounts are needed to let selection do its work.

It certainly would be more interesting to live in a world where selection pressures were accelerated by, say, an order of magnitude.

Posted by: Harry Eagar at March 18, 2004 4:57 PM

It would be interesting enough to live in one where they existed at all.

Posted by: oj at March 18, 2004 5:00 PM

Try giving your kids water from Chile, then come back and tell me that.

Posted by: Harry Eagar at March 18, 2004 6:56 PM

Chile is noted for its mineral water from the Andes.

Posted by: oj at March 18, 2004 8:28 PM

I saw a nature documentary once, that included a pack of jackels. One bitch fought for leadership of the pack, and won. Later that same day, she had her back broken by a lion.

If the strongest don't have a chance to breed, then natural selection would presumably favor the lucky, or perhaps, the quick-breeding, say bacteria.

Posted by: Michael Herdegen at March 19, 2004 4:08 AM

And a pack of bacteria would have killed Dr. Lysol before he came up with the Zyklon-B of bacteria.

Posted by: oj at March 19, 2004 8:15 AM
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