Chemistry Reference
In-Depth Information
photolysis of water and N
2
O to their component elements in the atmosphere by high-energy UV radiation, Oxygen
is consumed via respiration and decay, mechanisms by which animal and bacteria return carbon dioxide to the
atmosphere. Oxygen can also be lost because of chemical weathering of minerals at the surface of exposed rocks.
A good example is the formation of rust:
4Fe
þ
3O
2
/
2Fe
2
O
3
when marine organisms with calcium
carbonate (CaCO
3
) shells die, the shells are buried on the shallow waters of the sea floor, becoming the limestone
of the lithosphere. A small amount of atmospheric oxygen is transformed to ozone, O
3
, and the ozone layer in the
stratosphere plays an important role in shielding our planet from harmful ultraviolet radiation.
The flux of oxygen through these three pools are as follows. Photosynthesis accounts for about 300,000 Gt per
year, of which 55% is generated on land and 45% in the oceans. The contribution of photolysis is tiny (0.005%).
Respiration accounts for 94% of the total annual losses of around 300,000 Gt., with some 4% attributable to the
combustion of fossil fuel.
Phosphorus, the only one of the six major elements not to be involved in redox chemistry, is also unable to
access the atmosphere, since phosphorus itself and most phosphorus-based compounds are usually solids at typical
temperatures and pressures found on earth (only under highly reducing conditions is it found as the gas phosphine,
PH
3
). In a biological context, its principal roles are in the nucleotide di- and triphosphates like ATP, involved in
cellular energy transfer and in the nucleic acids DNA and RNA. It is also found in membranes as a component of
phospholipids, in bone, teeth, and insect exoskeleton, and it functions as an important buffering agent in many
biological fluids. And, we should not forget the important role played by phosphorylation/dephosphorylation
reactions in the regulation of intermediary metabolism.
Phosphorus is usually found in biological systems as the phosphate ion, which transits rapidly through plants
and animals, but moves much more slowly through the soil and the oceans, making the phosphorus cycle overall
one of the slowest biogeochemical cycles. The major mineral with an important phosphorus content is apatite
[Ca
5
(PO
4
)
3
OH], but this is not a major source, and many organisms rely on soil-derived phosphorus released from
dead organic matter for their phosphorus requirements.
Oxygen can also cycle between the biosphere and the lithosphere
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The Nitrogen Cycle
Because of its presence in both proteins and nucleic acids, the biological requirements for nitrogen, the 5th most
abundant element in the solar system, are enormous. For every 100 atoms of carbon incorporated into cells,
between 2 and 20 atoms of nitrogen are needed, depending on the organism (
Canfield, Glazer, & Falkowski, 2010
)
.
Nitrogen biogeochemistry is almost entirely dependent on redox reactions, mostly catalysed by metalloenzymes.
The nitrogen cycle, together with its associated enzymes, is shown in
Figure 18.5
.
The only reaction that makes the
extremely inert gas N
2
accessible for the synthesis of proteins and nucleic acids is catalysed by nitrogenase,
usually referred to as N
2
fixation. This highly conserved multi-enzyme complex converts N
2
to NH
4
.Aswesawin
Chapter 17, nitrogenase is made up of two proteins, the
a
2
b
2
heterotetrameric MoFe-protein containing both the
FeMo- cofactor and the P-cluster, and the homodimeric Fe-protein which binds a single [4Fe
4S] cluster at the
interface between the two subunits. Unlike most multiple electron transfer reactions, each of the eight individual
electron transfers between the Fe-protein and the MoFe-protein requires the binding and hydrolysis of two ATP
molecules. We know that some nitrogen fixers have alternative nitrogenases where Mo is replaced by Vor Fe, and
that these less efficient forms are expressed when Mo is unavailable. Given the abundant availability of soluble
Fe
2
þ
at the low atmospheric oxygen levels on the early Earth and the lack of soluble Mo under these conditions, it
is likely that the Fe form dominated at that point in evolution. Indeed, the more efficient Mo form may not have
become widely distributed until some 500
e
600 million years ago, when oxygenation of the deep ocean led to an
increase in soluble Mo concentration (
Canfield et al., 2010
). NH
4
can then be incorporated into amino acids and
purine and pyrimidine bases, and be assimilated by higher organisms from organic nitrogen in their food.
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