Lifes Little Essential Everybody knows liquid water is necessary for life, at least as we know it. But just why exactly? By Peter Tyson A friend of mine once had a poster on his office wall that asked at the top in big letters WHY IS THE SKY BLUE? I first saw the poster from a distance, and my initial reaction was to snicker slightly, thinking Everybody knows why the sky is blue. The rest of the poster proved to be the perfect rejoinder, for beneath that simple question lay row upon row of complex equations, originally published by Albert Einstein in 1911, that described in mathematical terms precisely why the sky is blue. When I looked beneath the surface of it, the question that opens this article elicited a similar effect: I was surprised by how much I didnt know about why water is thought necessary for life. Once I learned the particulars, it became clear why planetary scientists on the lookout for life on Mars and elsewhere in the solar system are on the lookout for water. OF THE ESSENCE So why is liquid water the sine qua non of life as we know it? Liquid water may sound redundant, but planetary scientists insist on using the qualifier, for solid or vaporous water wont do. The biochemical reactions that sustain life need a fluid in order to operate. In a liquid, molecules can dissolve and chemical reactions occur. And because a liquid is always in flux, it effectively conveys vital substances like metabolites and nutrients from one place to another, whether its around a cell, an organism, an ecosystem, or a planet. Getting molecules where they need to go is difficult within a solid and all too easy within a gas—vapor-based life would go all to pieces. And why is water the best liquid to do the job? For one thing, it dissolves just about anything. Water is probably the best solvent in the universe, says Jeffrey Bada, a planetary scientist at the Scripps Institution of Oceanography in La Jolla, Calif. Everything is soluble in water to some degree. Even gold is somewhat soluble in seawater. (Before you get any ideas about extracting gold from the oceans, I should add that, according to Bada, the value of dissolved gold in a metric ton of seawater comes to about $0.0000004). Water plays another key role in the biochemistry of life: bending enzymes. Enzymes are proteins that catalyze chemical reactions, making them occur much faster than they otherwise would. To do their handiwork, enzymes must take on a specific three-dimensional shape. Never mind how, but it is water molecules that facilitate this. BLACK SHEEP OF THE LIQUIDS Waters ability to so successfully further the processes of life has a lot to do with just how unusual a fluid it is. Not long ago, if I had to guess, I would have said that water is about as normal a liquid as they come. In fact, despite its ubiquity and molecular simplicity, H2O is abnormal in the extreme. Liquid water is still the Holy Grail for planetary scientists. For starters, while other substances form liquids, precious few do so under the conditions of temperature and pressure that prevail on our planets surface. In fact, next to mercury and liquid ammonia, water is our only naturally occurring inorganic liquid, the only one not arising from organic growth. It is also the only chemical compound that occurs naturally on Earths surface in all three physical states: solid, liquid, and gas. Good thing, otherwise the hydrological cycle that most living things rely on to ferry water from the oceans to the land and back again would not exist. As science journalist Philip Ball writes in his informative book Lifes Matrix: A Biography of Water, This cycle of evaporation and condensation has come to seem so perfectly natural that we never think to remark on why no other substances display such transformations. Compared to most other liquids, water also has an extremely large liquid range. Pure water freezes at 0°C (32°F) and boils at 100°C (212°F). Add salt and you can lower the freezing temperature; natural brines are known with freezing points below -50°F. Add pressure and you can raise the boiling temperature; deep-sea vent waters can reach over 650°F. Water also has one of the highest specific heats of any substance known, meaning it takes a lot of energy to raise the temperature of water even a few degrees. Waters broad liquid range and high heat capacity are good things, too. They mean that temperatures on the Earths surface, which is more than two-thirds water, can undergo extreme variations—between night and day, say, or between seasons—without its water freezing or boiling away, events that would throw a big wrench into life as we know it. As it is, the oceans serve as a powerful moderating influence on the worlds climate. Liquid water has yet another unusual property that means the difference between life and essentially no life in cold regions of the planet. Unlike most other liquids when they freeze, water expands and becomes less dense. Most other frozen liquids are denser than their melted selves and thus sink. If it sank, ice, being unable to melt because of the insulating layer of water above it, would slowly fill up lakes and oceans in cold climates, making sea life in those parts of the world a challenging prospect. Peter Tyson is editor in chief of NOVA Online. pbs.org/wgbh/nova/evolution/liquid-of-life.html
Posted on: Tue, 11 Mar 2014 04:22:00 +0000
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