Essential Cell Biology 5th edition

The Evolution of Energy-Generating Systems
489
redox potential (mV)
_ 400
_ 300
_ 200
light
produces
charge
separation
+ S 2
H 2 S
H +
photosystem
+
Fd
H + + NADP +
direction of electron flow
NADP +
reductase
NADPH
Figure 14–45 Photosynthesis in green
sulfur bacteria uses hydrogen sulfide
(H 2 S) as an electron donor rather than
water. Electrons are easier to extract from
H 2 S than from H 2 O, because H 2 S has a
much higher redox potential (compare
with Figure 14–39). Therefore, only one
photosystem is needed to produce NADPH,
and elemental sulfur is formed as a byproduct
instead of O 2 . The photosystem
in green sulfur bacteria resembles
photosystem I in plants and cyanobacteria.
These photosystems all use a series of
iron–sulfur centers as the electron carriers
that eventually donate their high-energy
electrons to ferredoxin (Fd). A bacterium of
this type is Chlorobium tepidum, which can
thrive at high temperatures and low light
intensities in hot springs.
maintain their internal pH. These cells could then generate large electrochemical
proton gradients, which they could couple to the production
of ATP (stage 3 in Figure 14–44). Because such cells would require much
less of the dwindling supply of fermentable nutrients, they would have
proliferated at the expense of their neighbors.
ECB5 e14.44/14.45
Photosynthetic Bacteria Made Even Fewer Demands on
Their Environment
The major evolutionary breakthrough in energy metabolism, however,
was almost certainly the formation of photochemical reaction centers
that could use the energy of sunlight to produce molecules such as
NADPH. It is thought that this development occurred early in the process
of evolution—more than 3 billion years ago, in the ancestors of
green sulfur bacteria. Present-day green sulfur bacteria use light energy
to transfer hydrogen atoms (as an electron plus a proton) from H 2 S to
NADPH, thereby creating the strong reducing power required for carbon
fixation (Figure 14–45).
The next step is thought to have involved the evolution of organisms
capable of using water instead of H 2 S as the electron source for photosynthesis.
This entailed the evolution of a water-splitting enzyme and the
addition of a second photosystem, acting in conjunction with the first, to
bridge the enormous gap in redox potential between H 2 O and NADPH
(see Figure 14–39).
The biological consequences of this evolutionary step were far-reaching.
For the first time, there were organisms that made only minimal chemical
demands on their environment. These cells—including the first cyanobacteria
(see Figure 14–28)—could spread and evolve in ways denied to
the earlier photosynthetic bacteria, which needed H 2 S, organic acids, or
other sources of electrons. Consequently, large amounts of fermentable
organic materials—produced by these cells and their ancestors—began
to accumulate. Moreover, O 2 began to enter the atmosphere in large
amounts (Figure 14–46).
The availability of O 2 made possible the development of bacteria that
relied on aerobic metabolism to make their ATP. As explained previously,
these organisms could harness the large amount of energy released when
carbohydrates and other reduced organic molecules are broken down all
the way to CO 2 and H 2 O.