The Sun, the Earth, and the Evolution of the Earth’s Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 40

to migrate to the top of the oceans and soil, ultimately
helping land plants to develop 430 to 395 m.y.a.
Around 1.85 b.y.a., oxygen levels reached about 1
percent of those today. Oxygen levels stayed relatively
constant at 1 percent for the next billion years, pos-
sibly because a new equilibrium had been reached
between oxygen production by photosynthesis and con-
sumption by recently evolved eukaryotic bacteria
(Section 2.3.5). About 850 m.y.a., freshwater green
algae evolved from some photosynthesizing eukary-
otic organisms, increasing oxygen levels further. How-
ever, not until 430 to 395 m.y.a. did land plants evolve
from algae, increasing oxygen levels more rapidly. Like
cyanobacteria and algae, plants photosynthesized to
produce oxygen. The spread of land plants resulted in
oxygen levels rising to those similar to today's, where
21 of every 100 molecules in the air are molecular
oxygen.
Oxygen-producing photosynthesis in plants is similar
to that in bacteria. In both cases, CO 2 (g) and sunlight are
required, and reactions occur in chlorophylls .Chloro-
phylls reside in photosynthetic membranes. In bacteria,
the membranes are cell membranes; in plants and algae,
photosynthetic membranes are found in chloroplasts .
Chlorophylls are made of pigments ,wh ch are
organic molecules that absorb visible light. Plant and
tree leaves generally contain two pigments, chlorophyll
a and b , both of which absorb blue wavelengths (shorter
than 500 nm) and red wavelengths (longer than 600 nm)
of visible light. Chlorophyll a absorbs red wavelengths
more efficiently than does chlorophyll b , and chloro-
phyll b absorbs blue wavelengths more efficiently than
does chlorophyll a .Because neither chlorophyll absorbs
in the green part of the visible spectrum (500-600 nm),
chlorophyll reflects green wavelengths, giving leaves a
green color. Photosynthetic bacteria generally appear
purple, blue, green, or yellow, indicating that their pig-
ments do not absorb purple, blue, or green, respectively,
but do absorb other colors.
The oxygen-producing photosynthesis process in
green plants is

Figure 2.9. Hot spring in Yellowstone National Park,
Wyoming. The hot, mineral-rich water provides ideal
conditions for colored photosynthetic cyanobacteria
to grow at the spring's perimeter, where the
temperatures drop to about 70 ◦ C. The colors identify
different photosynthetic bacteria with different
temperature optima. Photo by Alfred Spormann,
Stanford University.

of oxygen on Earth is bound in Fe 2 O 3 (s) rocks, 38 per-
cent is bound in SO 4 2 − rocks, and 4 percent is in the
air.
Around 2.45 b.y.a., rocks became saturated with oxy-
gen, so the oxygen slowly began to accumulate in the
atmosphere. Between 2.45 and 1.85 b.y.a., oxygen lev-
els increases from near zero to 1 percent of the compo-
sition of the air. The increase in oxygen at 2.45 b.y.a. is
referred to as the Great Oxygenation Event (GOE) .
The GOE had a significant effect on Earth's climate.
The increase in oxygen in the atmosphere increased
the photochemical production of oxygenated chemi-
cals, including the hydroxyl radical [OH(g)] and excited
atomic oxygen [O( 1 D)(g)]. These chemicals reacted
with CH 4 (g), a strong greenhouse gas, converting it to
CO 2 (g), a weaker one, triggering the Huronian glacia-
tion (Section 12.3.2.1) (Kasting and Siefert, 2002). The
slight oxygen buildup also caused a mass extinction of
anaerobic bacteria.
The GOE resulted in the first ozone layer .Oxygen
plus ultraviolet radiation produces ozone high above
the Earth's surface (Section 11.3), and both oxygen and
ozone help shield the Earth's surface from harmful UV
radiation. Molecular oxygen absorbs far UV radiation,
and ozone absorbs far UV, UV-C, and a large portion
of UV-B radiation. The gradually increasing abundance
of oxygen and ozone in the air protected the surface of
the Earth from UV radiation, allowing microorganisms

6CO 2 (g)
Carbon
dioxide

+ 6H 2 O(aq)

Liquid
water

+ h → C 6 H 12 O 6 (aq)

Glucose

+

6O 2 (g)
Molecular
oxygen

(2.10)

where the result, glucose, is dissolved in water in the
photosynthetic membrane of the plant. The source of
molecular oxygen during photosynthesis in green plants
is not carbon dioxide, but water. This can be seen by