Geoscience Reference
In-Depth Information
To probe this idea further, let's think of salt. Put salt in water and it
dissolves in a reaction that yields energy, but organisms cannot grow
from the energy of this reaction; no electrons are transferred; and the
chloride and sodium atoms have the same charge in the salt crystal as
they do in the solution. Now think of cows. Cows house enormous pop-
ulations of microbes in their digestive system, and many of them form
methane. Many of these microbes, so-called methanogens, grow quite
happily by combining hydrogen gas and carbon dioxide to form meth-
ane gas. No light is used, electrons are transferred, the methanogens are
happy, and so, presumably, are the cows. Therefore, a basic necessity for
life is energy, which is supplied either from light, or from a myriad of
different oxidation-reduction reactions.
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We will look at these issues in
more detail in the next chapter, but for now, it's sufficient to highlight
that energy is critical for life.
Energy is critical, but we need other things too. Cells are made up
of carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur as the
major ingredients, with a whole suite of trace metals and other elements
as well. All of these compounds are critical in the construction of basic
cellular components like the cell membrane, genetic material (DNA and
RNA), and all of the proteins and other molecules used in running the
cell's machinery.
Another basic ingredient of life, at least for life as we know it, is a
stable aqueous (meaning water) environment. Life likes it wet! Many
organisms, of course, have evolved to live outside of the watery sphere
of our planet, but they still all need water to live. So do we, but we just
pack it inside our bodies. So, whether we're talking about desert cacti,
spiders, snakes, trees, or the smallest bacteria, they all need water. In-
deed, this is one reason, as mentioned above, why the search for life
in our solar system and beyond is tantamount to searching for liquid
water. “Wait,” you might say, “I've heard about small bacteria and algae
living in sea ice and even in glacial ice in some cases.” Very true, but if
the organism is alive and growing,
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it has access to liquid water. In the
case of sea ice, this could be brine channels formed as salt is excluded
from the growing ice; or for glaciers, high pressure induces ice melting
near the bottom, providing an aqueous environment for organisms.
“Well then,” you might add, “I've heard that the temperature record for
a living organism is about 120°C (248°F), well above the boiling point
