Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
HERE BE DRAGONS<br />
Methanopyrus makes a living by taking molecular hydrogen (H 2 )<br />
and carbon dioxide (CO 2 ), both of which are present in the vent fluids,<br />
and making methane (CH 4 ) and water. The reaction can be written<br />
like this:<br />
This is an oxidation-reduction, or electron-transfer, reaction: electrons<br />
are being stripped from the hydrogen gas and donated to the carbon<br />
dioxide, thus reducing it. Protons come along for the ride, so the carbon<br />
atom ends up surrounded by uncharged hydrogen atoms rather<br />
than by negatively charged electrons. But the protons are pretty irrelevant<br />
from an energy-budget point of view because in any watery environment<br />
they are free for the taking—they are formed spontaneously<br />
by the dissociation of water molecules. Electrons, on the other hand,<br />
do not exist as free particles in water; rather, they must be transferred<br />
directly from donor to acceptor molecules. That is the reason that oxidation-reduction<br />
reactions in aqueous solution are usually defined as<br />
electron transfers rather than as hydrogen transfers.<br />
In this sense, beasts as different as Methanopyrus and Homo sapiens<br />
make their living in the same way. Humans survive by burning simple<br />
foodstuffs such as glucose (C 6 H 12 O 6 ) by the following reaction:<br />
in which electrons are transferred from the glucose to the oxygen.<br />
Organisms can in principle derive energy from any electron-transfer<br />
reaction, but only by running the reaction in the downhill direction—<br />
that is, the direction in which the free energy of the entire system (the<br />
energy available for doing work) is reduced. This, of course, is mandated<br />
by the second law of thermodynamics. The energy difference between<br />
the two states may be released as heat or, as we'll see in a moment,<br />
may be put to more constructive uses.<br />
Everett Shock of Washington University, St. Louis, has studied the<br />
chemical system in which Methanopyrus Jives. 7 The fluid rising<br />
through the vent is at a temperature of about 4OO°c. At this high temperature,<br />
the carbon dioxide and hydrogen are in equilibrium: no energy<br />
is to be gained by a reaction between them. As the fluid mixes<br />
with cold seawater, however, the temperature drops to a range<br />
(6o-25o°c) in which the gases are out of equilibrium: the conversion<br />
50
Change language