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

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Energy Conversion:
Mitochondria and Chloroplasts
chapter
14
To maintain their high degree of organization in a universe that is constantly drifting
toward chaos, cells have a constant need for a plentiful supply of ATP, as we
have explained in Chapter 2. In eukaryotic cells, most of the ATP that powers life
processes is produced by specialized, membrane-enclosed, energy-converting
organelles. These are of two types. Mitochondria, which occur in virtually all cells
of animals, plants, and fungi, burn food molecules to produce ATP by oxidative
phosphorylation. Chloroplasts, which occur only in plants and green algae, harness
solar energy to produce ATP by photosynthesis. In electron micrographs, the
most striking features of both mitochondria and chloroplasts are their extensive
internal membrane systems. These internal membranes contain sets of membrane
protein complexes that work together to produce most of the cell’s ATP. In
bacteria, simpler versions of essentially the same protein complexes produce ATP,
but they are located in the cell’s plasma membrane (Figure 14–1).
Comparisons of DNA sequences indicate that the energy-converting organelles
in present-day eukaryotes originated from prokaryotic cells that were endocytosed
during the evolution of eukaryotes (discussed in Chapter 1). This explains
why mitochondria and chloroplasts contain their own DNA, which still encodes
a subset of their proteins. Over time, these organelles have lost most of their own
genomes and become heavily dependent on proteins that are encoded by genes
in the nucleus, synthesized in the cytosol, and then imported into the organelle.
And the eukaryotic cells now rely on these organelles not only for the ATP they
need for biosynthesis, solute transport, and movement, but also for many important
biosynthetic reactions that occur inside each organelle.
The common evolutionary origin of the energy-converting machinery in mitochondria,
chloroplasts, and prokaryotes (archaea and bacteria) is reflected in the
fundamental mechanism that they share for harnessing energy. This is known
as chemiosmotic coupling, signifying a link between the chemical bond-forming
reactions that generate ATP (“chemi”) and membrane transport processes
In This Chapter
THE MITOCHONDRION
THE PROTON PUMPS OF THE
ELECTRON-TRANSPOrt CHAIN
ATP production in
mitochondria
chloroplasts and
photosynthesis
THE GENETIC SYSTEMS
OF MITOCHONDRIA AND
CHLOROPLASTS
bacterium
mitochondrion
cytoplasm
inner membrane
outer membrane
inner membrane
outer membrane
chloroplast
Figure 14–1 The membrane systems of
bacteria, mitochondria, and chloroplasts
are related. Mitochondria and chloroplasts
are cell organelles that have originated from
bacteria and have retained the bacterial
energy-conversion mechanisms. Like
their bacterial ancestors, mitochondria
and chloroplasts have an outer and an
inner membrane. Each of the membranes
colored in this diagram contains energyharvesting
electron-transport chains. The
deep invaginations of the mitochondrial
inner membrane and the internal
membrane system of the chloroplast
harbor the machinery for cellular respiration
and photosynthesis, respectively.
MBoC6 n14.301/14.01