Geoscience Reference
In-Depth Information
O
O
O
O
C
C
C
C
C
C
N
N
D -alanine
L -alanine
Figure 8.3
Example of chirality: alanine, which is an amino acid. The two enantiomers are mirror images of
each other and cannot be superposed by any rotation. Unlabelled atoms are hydrogen. Biological
alanine is L .
but cannot be superposed by rotation ( Fig. 8.3 ). These two configurations, known as enan-
tiomers, have a different effect on polarized light and are prefixed by D , for dextrogyre,
for rotating light to the right, or L (levogyre) otherwise. Biomolecules are either non-chiral
or 100 percent L or D (homochirality), most sugars being D and 19 out of the 20 amino
acids being L (glycine is not chiral). The mechanism that produced homochirality in the
first place is relevant to the origin of life but is not understood.
Some additional defining effects of biogeochemistry may be summarized as follows:
1. The most abundant elements in the biomass are reduced carbon, hydrogen, nitrogen, and
phosphate. Fossil biomolecules are of particular economic importance since, as every
living creature, humans like to turn reduced carbon into energy.
2. Biological activity affects sediment composition through the deposition of biominerals
(carbonate, phosphate) or by interfering with diagenesis.
3. Over the Earth's history, biological activity has modified the composition of the atmo-
sphere (notably by adding oxygen and removing carbon dioxide) and of the ocean. It is
still today an essential parameter in climatic regulation.
4. Biological activity maintains chemical gradients such as those of nutrients in the water
column.
5. Some of the original biomolecules, mostly insoluble lipids, are left untouched by
diagenesis. They are true geochemical fossils and as such are called biomarkers.
We will now address some of these properties.
8.3 The chemistry of life
Living creatures can be divided into eukaryotes, which shelter their genes in a nucleus
(e.g. plants and animals), and prokaryotes with no nuclear membrane. The second group
is clearly divided by genetic analysis into archæa, which often live in unusual or extreme
 
 
 
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