Geoscience Reference
In-Depth Information
remarkable absorption power of the ozone layer is shown by the fact
that UV-B radiation at ground level is 350 million times weaker than
that above the ozone layer in the upper atmosphere. Even with that
natural protection, the UV-B radiation that reaches the Earth's surface
continually damages the DNA of living organisms, which causes
cancers and other cell abnormalities when damage occurs at a rate that
is faster than the cells' abilities to repair their DNA. Without the UV
protection provided by the ozone layer, most organisms that currently
live on the surface of the Earth could not survive there.
4.5. Nitrogen: a chemical element over which countries have
fought in the past
Nitrogen is an essential constituent of amino acids and proteins.
Amino acids themselves are major constituents of proteins, which
determine the structure and function of living organisms. Many proteins
are enzymes, which catalyze biochemical metabolic reactions. Nitrogen
is also a constituent of nucleic acids (RNA and DNA), which are the
most complex molecules that exist in nature. Nucleic acids carry
instructions that encode all biological molecules and transmit genetic
information from cell to cell during cell division (Figure 4.2).
Moreover, nitrogen is present in chlorophyll, the pigment of
photosynthesis (sections 4.3 and 4.4). Hence nitrogen is an essential
chemical element for life on Earth and a key element in
biogeochemical cycles.
4.5.1.
Nitrogen, abundant but difficult to access
On Earth, nitrogen is found in many natural reservoirs. It is much
less abundant than carbon or oxygen in the lithosphere, ocean and
biosphere, but much more abundant than these two chemical elements
in the atmosphere (Table 4.3). In fact, gaseous nitrogen (N
2
) accounts
for 75% of the mass and almost 80% of the volume of the Earth's
atmosphere (Table 4.2 and section 4.1.2). Given the abundance of this
chemical element in the atmosphere, one could assume that living
