Environmental Engineering Reference
In-Depth Information
THE CARBON ATOM AND ITS CHEMISTRY
Carbon can exist in various compounds in different oxidation states: all states from
4
to
4 are possible. From the point of view of a C atom, the oxidation state increases
(becomes more positive or less negative) if that atom is bound to either more O atoms,
fewer H atoms, or both. For example, CH
4
is a highly reduced form of C (oxidation state
is
1
4). Organic matter is always
substantially more reduced than CO
2
and moderately more oxidized than CH
4
. To move
C from a more oxidized to a more reduced state requires the input of energy; to move it to
a more oxidized state releases energy (as long as an appropriate chemical oxidant, also
called an electron acceptor, is present). With sufficient activation energy, reduced forms of C
(e.g., CH
4
, oil, sugars) will be oxidized in the presence of O
2
and some other electron
acceptors (e.g., SO
4
2
2
,NO
3
2
,Fe
1
3
). Biological enzymes, by reducing the amount of activa-
tion energy required for certain redox reactions to occur, catalyze many very important
biogeochemical fluxes that would otherwise occur at much slower rates.
4), whereas CO
2
is highly oxidized (oxidation state is
1
Key Biologic Reactions in the Carbon Cycle
The reduction of CO
2
to organic matter using light energy (photosynthesis) or chemical
energy (chemosynthesis) is the source of essentially all organic matter on Earth (see
Chapter 2). The oxidation of inorganic compounds in chemosynthesis generates the energy
that some organisms use to reduce CO
2
to organic C compounds. The oxidation of organic
C (catabolic processes) provides the energy for most heterotrophic organisms.
Carbon is involved in nearly every energy-generating and most energy-consuming reac-
tions in biology. Outlined next are some of the most important energetic reactions in the
biosphere.
Oxygenic Photosynthesis
This is the major photosynthetic pathway for modern plants and photosynthetic micro-
organisms. In oxygenic photosynthesis, O
2
is generated and CO
2
is reduced to organic
compounds; light is the energy source and water is the electron (hydrogen) donor.
Oxygenic photosynthesis first appeared on Earth about 2 billion years before present
(YBP) with the evolution of
the cyanobacteria. A general stoichiometric scheme for
oxygenic photosynthesis is:
CO
2
1
H
2
O
light
! ð
CH
2
O
Þ
1
O
2
ð
6
1
Þ
1
:
where CH
2
O represents the initial carbohydrate products of photosynthesis.
Anoxygenic Photosynthesis
Anoxygenic photosynthesis creates organic C from CO
2
without generating O
2
. This
was the earliest type of photosynthesis on Earth and is present today in major bacterial
groups such as the purple sulfur bacteria. This is another form of CO
2
fixation, in which
light is again the energy source but compounds other than water (e.g., H
2
or H
2
S) are
