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

798 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
CHLOROPLASTS
MITOCHONDRIA
EUKARYOTES
PROKARYOTES
cyanobacteria
(e.g., Synechococcus)
β-proteobacteria
(e.g., Bordetella)
α-proteobacteria
(e.g., Agrobacterium)
γ-proteobacteria
(e.g., E. coli)
purple nonsulfur
bacteria
green filamentous
bacteria
OXIDIZING
ATMOSPHERE
REDUCING
ATMOSPHERE
O 2 respiration
H 2 O photosynthesis
purple sulfur
bacteria
green sulfur
bacteria
H 2 S photosynthesis
ancestral
fermenting bacteria
Summary
Figure 14–57 Evolutionary scheme showing the postulated origins of mitochondria and chloroplasts and their bacterial
ancestors. The consumption of oxygen by respiration is thought to have first developed about 2 billion years ago. Nucleotidesequence
analyses suggest that an endosymbiotic oxygen-evolving cyanobacterium (cyan) gave rise to chloroplasts (dark
green), while mitochondria arose from an α-proteobacterium. The nearest relatives of mitochondria (pink) are members of
three closely related groups of α-proteobacteria—the rhizobacteria, agrobacteria, and rickettsias—known to form intimate
associations with present-day eukaryotic cells. Proteobacteria are pink, purple photosynthetic bacteria are purple, and other
photosynthetic bacteria are light green.
MBoC6 m14.72/14.57
Chloroplasts and photosynthetic bacteria have the unique ability to harness the
energy of sunlight to produce energy-rich compounds. This is achieved by the photosystems,
in which chlorophyll molecules attached to proteins are excited when
hit by a photon. Photosystems are composed of an antenna complex that collects
solar energy and a photochemical reaction center, in which the collected energy is
funneled to a chlorophyll molecule held in a special position, enabling it to withdraw
electrons from an electron donor. Chloroplasts and cyanobacteria contain
two distinct photosystems. The two photosystems are normally linked in series in
the Z scheme, and they transfer electrons from water to NADP + to form NADPH,
generating a transmembrane electrochemical potential. One of the two photosystems—photosystem
II—can split water by removing electrons from this ubiquitous,
low-energy compound. All the molecular oxygen (O 2 ) in our atmosphere is
a by-product of the water-splitting reaction in this photosystem. The three-dimensional
structures of photosystems I and II are strikingly similar to the photosystems
of purple photosynthetic bacteria, demonstrating a remarkable degree of conservation
over billions of years of evolution.
The two photosystems and the cytochrome b 6 -f complex reside in the thylakoid
membrane, a separate membrane system in the central stroma compartment of the