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
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chloroplast that is differentiated into stacked grana and unstacked stroma thylakoids.
Electron-transport processes in the thylakoid membrane cause protons to be
released into the thylakoid space. The backflow of protons through the chloroplast
ATP synthase then generates ATP. This ATP is used in conjunction with the NADPH
produced by photosynthesis to drive a large number of biosynthetic reactions in
the chloroplast stroma, including the carbon-fixation cycle, which generates large
amounts of carbohydrates from CO 2 .
In the early evolution of life, cyanobacteria overcame a major obstacle in devising
a way to use solar energy to split water and fix carbon dioxide. Cyanobacteria
produced both abundant organic nutrients and molecular oxygen, enabling the rise
of a multitude of aerobic life-forms. The chloroplasts in plants have evolved from
a cyanobacterium that was endocytosed long ago by an aerobic eukaryotic host
organism.
NADH NAD + reverse
electron
NADH
dehydrogenase
flow
Q
H + H +
b-c
complex
cyt
c 2
+
light
produces
charge
separation
PURPLE NONSULFUR BACTERIA
NADP + NADPH
Q
light
b-f
complex
produces
charge
light
separation
pC
produces
+
charge
separation
H 2 O +
PLANT CHLOROPLASTS AND CYANOBACTERIA
NADH
dehydrogenase
H +
Q
cyt c
reductase
cyt c
C
cytochrome c
oxidase
MITOCHONDRIA
O 2
H 2 O
Figure 14–58 A comparison of three electron-transport chains discussed in this chapter.
Bacteria, chloroplasts, and mitochondria all contain a membrane-bound enzyme complex that
resembles the cytochrome c reductase of mitochondria. These complexes all accept electrons from
a quinone carrier (Q) and pump H + across their respective membranes. Moreover, in reconstituted
in vitro systems, the different complexes can substitute for one another, and the structures of
MBoC6 m14.73/14.58
their protein components reveal that they are evolutionarily related. Note that the purple nonsulfur
bacteria use a cyclic flow of electrons to produce a large electrochemical proton gradient that drives
a reverse electron flow through NADH dehydrogenase to produce NADH from NAD + + H + + e – .