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

794 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
EXTRACELLULAR
SPACE
CYTOSOL
(A)
PURPLE BACTERIA
THYLAKOID
SPACE
LHC-II
STROMA
(B)
PHOTOSYSTEM II
THYLAKOID
SPACE
STROMA
(C)
PHOTOSYSTEM I
LH1
2 H 2 O
M L LH1
Mn
D 1 D 2
core antenna protein
Psa A
Q
Q
Q
pC
Q
Q
Q
cytochrome
O 2 + 4 H +
plastocyanin
Psa B
LHC-II
Figure 14–53 Evolution of
photosynthetic reaction centers.
Pigments involved in light-harvesting are
colored green; those involved in the central
photochemical events are colored red.
(A) The primitive photochemical reaction
center of purple bacteria contains two
related protein subunits, L and M, that
bind the pigments involved in the
central process of photosynthesis,
including a special pair of chlorophyll
molecules. Electrons are fed into the
excited chlorophylls by a cytochrome.
LH1 is a bacterial antenna complex.
(B) Photosystem II contains the D 1 and
D 2 proteins, which are homologous to
the L and M subunits in (A). The excited
P 680 chlorophyll in the special pair
withdraws electrons from water held by
the manganese cluster. LHC-II is the lightharvesting
complex that feeds energy into
the core antenna proteins. (C) Photosystem
I contains the Psa A and Psa B proteins,
each of which is equivalent to a fusion of
the D 1 or D 2 protein to a core antenna
protein of photosystem II. The loosely
bound plastocyanin (pC) feeds electrons
into the excited chlorophyll pair. As
indicated, in photosystem I, electrons are
passed from a bound quinone (Q) through
a series of three iron–sulfur centers (red
circles). (Modified from K. Rhee, E. Morris,
J. Barber and W. Kühlbrandt, Nature
396:283–286, 1998; and W. Kühlbrandt,
Nature 411:896–899, 2001. With
permission from Macmillan Publishers Ltd.)
The Proton-Motive Force for ATP Production in Mitochondria and
Chloroplasts Is Essentially the Same
The proton gradient across the thylakoid membrane depends both on the proton-pumping
activity of the cytochrome b 6 -f complex and on the photosynthetic
activity of the two photosystems, which in turn depends on light intensity. In chloroplasts
exposed to light, H + is MBoC6 pumped m14.50/14.53 out of the stroma (pH around 8, similar to
the mitochondrial matrix) into the thylakoid space (pH 5–6), creating a gradient of
2–3 pH units across the thylakoid membrane, representing a proton-motive force
of about 180 mV. This is very similar to the proton-motive force in respiring mitochondria.
However, a membrane potential across the inner mitochondrial membrane
makes the largest contribution to the proton-motive force that drives the
mitochondrial ATP synthase to make ATP, whereas a H + gradient predominates
for chloroplasts.
In contrast to mitochondrial ATP synthase, which forms long rows of dimers
along the cristae ridges, the chloroplast ATP synthase is monomeric and located
in flat membrane regions (Figure 14–54). Evidently, the H + gradient across the
thylakoid membrane is high enough for ATP synthesis without the need for the
elaborate arrangement of ATP synthase seen in mitochondria.
Chemiosmotic Mechanisms Evolved in Stages
The first living cells on Earth may have consumed geochemically produced
organic molecules and generated their 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