Essential Cell Biology 5th edition

490 CHAPTER 14 Energy Generation in Mitochondria and Chloroplasts
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
oceans and
continents
formation
of the Earth
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 14–46 Oxygen entered Earth’s
atmosphere billions of years ago. With the
evolution of photosynthesis in prokaryotes
more than 3 billion years ago, organisms
would have no longer depended on
preformed organic chemicals: they could
make their own organic molecules from
CO 2 . Note that there was a delay of about
a billion years between the appearance
of photosynthetic bacteria that split water
and released O 2 and the accumulation of
high levels of O 2 in the atmosphere. This
delay is thought to have been due to the
initial reaction of the O 2 with abundant
ferrous iron (Fe 2+ ) dissolved in the early
oceans. Only when the iron was used
up could large amounts of O 2 begin to
accumulate in the atmosphere. In response
to rising amounts of O 2 in the atmosphere,
nonphotosynthetic, aerobic organisms
appeared, and the concentration of O 2 in
the atmosphere eventually leveled out.
As organic materials accumulated as a by-product of photosynthesis,
some photosynthetic bacteria—including the ancestors of the bacterium
Escherichia coli—lost their ability to survive on light energy alone and
came to ECB5 rely entirely m14.56/14.46 on cell respiration. Mitochondria arose when a preeukaryotic
cell engulfed such an aerobic bacterium (see Figure 1–19).
Plants arose somewhat later, when a descendant of this early aerobic
eukaryote captured a photosynthetic bacterium, which became the precursor
of chloroplasts (see Figure 1–21). Once eukaryotes had acquired
the bacterial symbionts that became mitochondria and chloroplasts, they
could then embark on the spectacular pathway of evolution that eventually
led to complex multicellular organisms, including ourselves.
The Lifestyle of Methanococcus Suggests That
Chemiosmotic Coupling Is an Ancient Process
The conditions today that most resemble those under which cells are
thought to have lived 3.5–3.8 billion years ago may be those near deepocean
hydrothermal vents. These vents represent places where the
Earth’s molten mantle is breaking through the overlying crust, expanding
the width of the ocean floor. Indeed, the modern organisms that appear
to be most closely related to the hypothetical cells from which all life
evolved live at 75°C to 95°C, temperatures approaching that of boiling
water. This ability to thrive at such extreme temperatures suggests that
life’s common ancestor—the cell that gave rise to bacteria, archaea, and
eukaryotes—lived under very hot, anaerobic conditions.
One of the archaea that live in this environment today is Methanococcus
jannaschii. Originally isolated from a hydrothermal vent more than a
mile beneath the ocean surface, the organism grows in the complete
absence of light and gaseous oxygen, using as nutrients the inorganic
gases—hydrogen (H 2 ), CO 2 , and nitrogen (N 2 )—that bubble up from the
vent (Figure 14–47). Its mode of existence gives us a hint of how early
cells might have used electron transport to derive energy and to extract
carbon molecules from inorganic materials that were freely available on
the hot early Earth.
Methanococcus relies on N 2 gas as its source of nitrogen for making
organic molecules such as amino acids. The organism reduces N 2 to
ammonia (NH 3 ) by the addition of hydrogen, a process called nitrogen
fixation. Nitrogen fixation requires a large amount of energy, as does the
carbon-fixation process that converts CO 2 and H 2 O into sugars. Much