Geoscience Reference
In-Depth Information
of water at Earth's surface.” True again, but these organisms are only
found at high pressures, like deep in the ocean where the boiling point
of water exceeds the upper temperature limit for life.
hat is the big deal about water anyway? For one, water has special
properties. Because of its physical structure, a water molecule is bipolar,
which means that it is slightly charged with a positive charge on one side
and a negative charge on the opposite side. This condition allows it to
dissolve all kinds of so-called ionic chemical substances (also charged),
many of which constitute the building blocks of life. These include nu-
trients like nitrate, ammonium, and phosphate, which form into critical
components of DNA, RNA, and cell membranes, as well as a host of
other substances including sulfate and a variety of trace metals, which
help to build the biochemical machinery of the cell. Not only does water
dissolve the substances, but these substances are also transported by
diffusion and advection; and this movement provides a means by which
they can be supplied to the cells. Water also provides the medium by
which waste products can be exported from the cell.
The bipolar nature of water also allows for the formation of cell mem-
branes. These separate the external environment from the inside of the
cell where the business of life is conducted. Cell membranes are made
up of special (phospholipid) molecules with one end containing water-
loving chemical groups (hydrophilic) and the other end containing
water-repelling chemical groups (hydrophobic). In forming a mem-
brane, the water-loving side reaches out toward the water phase, while
the water-repelling side reaches in and lies foot to foot with another row
of water repelling bits whose water-loving sides reach out in the oppo-
site direction. This lipid bilayer joins in a circle forming the cell mem-
brane, separating the inside of the cell from the outside environment.
All in all, from its ability to dissolved and transport the chemical con-
stituents of life, to its ability to host membrane structures, water is a
unique chemical substance.
Or maybe we're thinking too small, too Earthcentrically. Water is the
fluid of life because its properties are perfect for the type of life that we
know. Perhaps a different type of life could have evolved in different
solvents with different properties. It's hard to rule this possibility out.
Alternative potential solvents are sometimes named. These include am-
