Geoscience Reference
In-Depth Information
first dividing Reaction 2.10 by 6, and then adding water
to each side of the equation. The result is
Table 2.5.
Sources and sinks of atmospheric
molecular oxygen
CO
2
(g)
Carbon
dioxide
+
2H
2
O(aq)
Liquid
water
+
h
→
CH
2
O (aq)
Carbo
−
hydrate
Sources
Sinks
Photosynthesis by green
plants and
cyanobacteria
Photolysis and kinetic
reaction
Aerobic respiration
+
H
2
O(aq)
Liquid
water
+
O
2
(g)
Molecular
oxygen
(2.11)
Atmospheric chemical
reaction
Dissolution into ocean water
Rusting
Chemical reaction on soil
and rock surfaces
Fossil fuel, biofuel, and
biomass burning
A comparison of Reaction 2.11 with Reaction 2.9 indi-
cates that because the source of atomic sulfur in Reac-
tion 2.9 is hydrogen sulfide, the analogous source of
oxygen in Reaction 2.11 should be water. This was
first hypothesized in 1931 by Cornelius B. Van Niel, a
Dutch microbiologist working at Stanford University,
and later proved to be correct experimentally with the
use of isotopically labeled water.
cell component, occurs by
2.3.5. Aerobic Respiration and
the Oxygen Cycle
The atmospheric production of molecular oxygen and
ozone 2.45 b.y.a. resulted in biological changes in
organisms that shaped our present atmosphere. Most
important was the development of
aerobic respiration
,
which is the process by which molecular oxygen reacts
with organic cell material to produce energy during cel-
lular respiration.
Cellular respiration
is the oxidation
of organic molecules in living cells.
Whereas aerobic respiration may have developed
first in prokaryotes (bacteria and blue-green algae), its
spread coincided with the rise of another type of organ-
ism, the
eukaryote
, about 2.1 to 1.85 b.y.a. A eukaryotic
cell contains DNA surrounded by a true membrane-
enclosed nucleus. This differs from a prokaryotic cell,
which contains a single strand of DNA but not a nucleus.
Unlike prokaryotes, many eukaryotes became multicel-
lular. Today, the cells of all higher animals, plants, fungi,
protozoa, and most algae are eukaryotic. Prokaryotic
cells never evolved past the microbial stage.
Almost all eukaryotic cells respire aerobically. In
fact, such cells usually switch from fermentation to
aerobic respiration when oxygen concentrations reach
about 1 percent of the present oxygen level (Pollack and
Yung, 1980). Thus, eukaryotic cells probably developed
substantially only when oxygen's atmospheric mixing
ratio increased to 1 percent of its present level. This
occurred about 1.85 b.y.a., after oxygen had became
substantially saturated in rocks and had started to accu-
mulate in the atmosphere.
The products of aerobic respiration are carbon diox-
ide and water. Aerobic respiration of glucose, a typical
C
6
H
12
O
6
(aq)
Glucose
+
6O
2
(g)
Molecular
oxygen
→
6CO
2
(g)
Carbon
dioxide
+
6H
2
O(aq)
Liquid
water
(2.12)
This process produces energy more efficiently than does
fermentation or anaerobic respiration. Thus, Reaction
2.12 was an evolutionary improvement.
Table 2.5 summarizes the current sources and sinks of
O
2
(g). The primary source is photosynthesis. The major
sinks are photolysis in and above the stratosphere and
aerobic respiration.
2.3.6. The Nitrogen Cycle
The development of aerobic respiration hastened the
evolution of organisms affecting the nitrogen cycle.
This cycle centers on molecular nitrogen [N
2
(g)], which
comprises about 78 percent of total air by volume today.
Figure 2.10 summarizes the major processes in the
nitrogen cycle. Four of the five processes are carried
out by bacteria in soils. The fifth involves nonbiologi-
cal chemical reactions in the air.
The direct source of almost all molecular nitrogen
in the air today is
denitrification
,thetwo-step pro-
cess carried out by anaerobic bacteria in soils that
was described by Reactions 2.7 and 2.8. Denitrification
evolved around 3.2 b.y.a., prior to the buildup of oxygen.
Whereas the second step of denitrification can produce
either nitric oxide [NO(g)], nitrous oxide [N
2
O(g)], or
N
2
(g), N
2
(g) is the dominant product. N
2
(g) is also pro-
duced chemically from N
2
O(g), which can form from
NO(g).
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