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
795
Figure 14–54 A comparison of H
MITOCHONDRION
+
concentrations and the arrangement
of ATP synthase in mitochondria and
chloroplasts. In both organelles, the pH in
matrix
the intermembrane space is 7.4, as in the
H +
H +
pH 7.9
cytoplasm. The pH of the mitochondrial
matrix and the pH of the chloroplast stroma
H +
P i
are both about 8 (light gray). The pH in the
+
intermembrane space thylakoid space is around 5.5, depending
ADP
on photosynthetic activity. This results
pH 7.4
H + in a high proton-motive force across the
ATP
thylakoid membrane, consisting largely of
the H + gradient (a high permeability of this
membrane to Mg 2+ and Cl – ions allows the
flow of these ions to dissipate most of the
membrane potential).
cristae
In contrast to chloroplasts, the H +
CHLOROPLAST
thylakoid membrane
gradient across the inner mitochondrial
membrane is insufficient for atp production,
and mitochondria need a membrane
stroma
potential to bring the proton-motive force
pH 8
to the same level as in chloroplasts. The
H + arrangement of the mitochondrial atp
synthase in rows of dimers along the
ATP ADP + P i
cristae ridges (see Figure 14–32) next to the
thylakoid space
respiratory-chain proton pumps may help
pH 5.5
the flow of protons along the membrane
surface toward the atp synthase, as the
H + H+ H + H+ H + H + H +
availability of protons is limiting for atp
intermembrane space
production. In the chloroplast, the atp
pH 7.4
synthase is distributed randomly in thylakoid
membranes.
H+ H + MBoC6 n14.328/14.54
primitive
bacterium
ATP-driven
proton pump
environment (see Figure 2–47). Perhaps such acids lowered the pH of the environment,
favoring the survival of cells that evolved transmembrane proteins that
could pump H + out of the cytosol, thereby preventing the cell from becoming too
acidic (stage 1 in Figure 14–55). One of these pumps may have used the energy
available from ATP hydrolysis to eject H + from the cell; such a proton pump could
have been the ancestor of present-day ATP synthases.
As the Earth’s supply of geochemically produced nutrients began to dwindle,
organisms that could find a way to pump H + without consuming ATP would have
been at an advantage: they could save the small amounts of ATP they derived
from the fermentation of increasingly scarce foodstuffs to fuel other important
activities. This need to conserve resources might have led to the evolution of
electron-transport proteins that allowed cells 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 14–55). Some of these cells
might have used the nonfermentable organic acids that neighboring cells had
excreted as waste to provide the electrons needed to feed this electron-transport
system. Some present-day bacteria grow on formic acid, for example, using the
small amount of redox energy derived from the transfer of electrons from formic
acid to fumarate to pump H + .
Figure 14–55 How ATP synthesis by chemiosmosis might have evolved
in stages. The first stage could 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 would then have linked these two systems
together to generate a primitive atp synthase that used the protons pumped
by the electron-transport chain to synthesize atp. An early bacterium with
this final system would have had a selective advantage over bacteria with
neither of the systems or only one.
STAGE 1
H + H +
e –
STAGE 2
ATP-driven proton pump
working in reverse to
make ATP
H + H + H +
e –
ADP + P i ATP
STAGE 3
ATP
ADP + P i
electron-transport
protein that pumps
protons