Essential Cell Biology 5th edition

488 CHAPTER 14 Energy Generation in Mitochondria and Chloroplasts
The glyceraldehyde 3-phosphate exported from chloroplasts into the
cytosol can also be converted into many other metabolites, including the
disaccharide sucrose. Sucrose is the major form in which sugar is transported
between the cells of a plant: just as glucose is transported in the
blood of animals, so sucrose is exported from the leaves via the vascular
system to provide carbohydrate to the rest of the plant.
THE EVOLUTION OF ENERGY-GENERATING
SYSTEMS
The ability to sequence the genomes of microorganisms that are difficult,
if not impossible, to grow in culture has made it possible to identify a
huge variety of previously mysterious life-forms. Some of these unicellular
organisms thrive in the most inhospitable habitats on the planet,
including sulfurous hot springs and hydrothermal vents that lie deep on
the ocean floor. In these remarkable microbes, we are finding clues to
life’s history. Like fingerprints left at the scene of a crime, the proteins and
small molecules these organisms produce provide evidence that allows
us to trace the history of ancient biological events, including those that
gave rise to the ATP-generating systems present in the mitochondria and
chloroplasts of modern eukaryotic cells. We therefore end this chapter
with a brief review of what has been learned about the origins of presentday
energy-harvesting systems, which have played such a critical part in
fueling the evolution of life on Earth.
primitive
cell
STAGE 1
STAGE 2
ADP + P
STAGE 3
ATP
e –
ATP
ATP-driven
proton pump
ADP + P
H + H +
ATP-driven proton pump
working in reverse to
make ATP
H + H + H +
e –
electron-transport
protein that pumps
protons
Oxidative Phosphorylation Evolved in Stages
As we mentioned earlier, the first living cells on Earth may have consumed
geochemically produced organic molecules and generated ATP
by fermentation. Because oxygen was not yet present in the atmosphere,
such anaerobic fermentation reactions would have dumped organic
acids—such as lactic or formic acids, for example—into the environment
(see Figure 13−6A).
A buildup of such acids would have lowered the pH of the environment,
favoring the survival of cells that evolved transmembrane proteins that
could pump H + out of the cytosol, preventing the cell interior from becoming
too acidic. Some of these pumps may have used the energy available
from ATP hydrolysis to eject H + from the cell (stage 1 in Figure 14–44).
Such a proton pump could have been the ancestor of present-day ATP
synthases. Other pumps, like those in modern respiratory chain complexes,
eventually evolved to use the movement of electrons between
molecules of different redox potentials as a source of energy for pumping
H + across the plasma membrane (stage 2 in Figure 14–44). Indeed, some
present-day bacteria that grow on formic acid use the small amount of
redox energy derived from the transfer of electrons from formic acid to
fumarate to pump H + .
When these H + -pumping electron-transport systems became efficient
enough, cells could harvest more redox energy than they needed to
Figure 14–44 Chemiosmotic processes most likely evolved in
stages. The first stage might have involved the evolution of an ATPase
that pumped protons out of the cell using the energy of ATP hydrolysis.
Stage 2 could have involved the evolution of a different proton pump,
driven by an electron-transport chain. Stage 3 could then link these two
systems together to generate an ATP synthase that uses the protons
pumped by the electron-transport chain to synthesize ATP. An early cell
with this final system would have had a large selective advantage over
cells with neither of the systems or only one.