Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CHLOROPLASTS AND PHOTOSYNTHESIS
797
OXYGEN
LEVELS IN
ATMOSPHERE
(%)
20
10
start of rapid O 2
accumulation
first land plants
TIME
(BILLIONS
OF YEARS)
4
3
2
1
formation
of the Earth
formation of
oceans and
continents
first
living
cells
first photosynthetic
cells
first water-splitting
photosynthesis
releases O 2
aerobic
respiration
becomes
widespread
origin of eukaryotic
photosynthetic cells
first vertebrates
first multicellular plants
and animals
present day
Figure 14–56 Major events during the evolution of living organisms on Earth. With the evolution of the membrane-based
process of photosynthesis, organisms were able to make their own organic molecules from CO 2 gas. The delay of more than
10 9 years between the appearance of bacteria that split water and released O 2 during photosynthesis and the accumulation
of high levels of O 2 in the atmosphere is thought to be due to the initial reaction of the oxygen with the abundant ferrous iron
(Fe 2+ ) that was dissolved in the early oceans. Only when the ferrous iron was used up would oxygen have started to accumulate
in the atmosphere. In response to the rising MBoC6 oxygen m14.71/14.56
levels, nonphotosynthetic oxygen-consuming organisms evolved, and the
concentration of oxygen in the atmosphere equilibrated at its present-day level.
The increase in atmospheric O 2 was very slow at first and would have allowed
a gradual evolution of protective devices. For example, the early seas contained
large amounts of iron in its reduced, ferrous state (Fe 2+ ), and nearly all the O 2 produced
by early photosynthetic bacteria would have been used up in oxidizing Fe 2+
to ferric Fe 3+ . This conversion caused the precipitation of huge amounts of stable
oxides, and the extensive banded iron formations in sedimentary rocks, beginning
about 2.7 billion years ago, help to date the spread of the cyanobacteria. By
about 2 billion years ago, the supply of Fe 2+ was exhausted, and the deposition
of further iron precipitates ceased. Geological evidence reveals how O 2 levels in
the atmosphere have changed over billions of years, approximating current levels
only about 0.5 billion years ago (Figure 14–56).
The availability of O 2 enabled the rise of bacteria that developed an aerobic
metabolism to make their ATP. These organisms could harness the large amount
of energy released by breaking down carbohydrates and other reduced organic
molecules all the way to CO 2 and H 2 O, as explained when we discussed mitochondria.
Components of preexisting electron-transport complexes were modified
to produce a cytochrome oxidase, so that the electrons obtained from organic
or inorganic substrates could be transported to O 2 as the terminal electron acceptor.
Some present-day purple photosynthetic bacteria can switch between photosynthesis
and respiration depending on the availability of light and O 2 , with only
relatively minor reorganizations of their electron-transport chains.
In Figure 14–57, we relate these postulated evolutionary pathways to different
types of bacteria. By necessity, evolution is always conservative, taking parts of
the old and building on them to create something new. Thus, parts of the electron-transport
chains that were derived to service anaerobic bacteria 3–4 billion
years ago survive, in altered form, in the mitochondria and chloroplasts of today’s
higher eukaryotes. A good example is the overall similarity in structure and function
between the cytochrome c reductase that pumps H + in the central segment of
the mitochondrial respiratory chain and the analogous cytochrome b-f complex
in the electron-transport chains of both bacteria and chloroplasts, revealing their
common evolutionary origin (Figure 14–58).