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

782 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
Summary
The large amount of free energy released when H + flows back into the matrix from
the cristae provides the basis for ATP production on the matrix side of mitochondrial
cristae membranes by a remarkable protein machine—the ATP synthase. The
ATP synthase functions like a miniature turbine, and it is a reversible device that can
couple proton flow to either ATP synthesis or ATP hydrolysis. The transmembrane
electrochemical gradient that drives ATP production in mitochondria also drives
the active transport of selected metabolites across the inner mitochondrial membrane,
including an efficient ADP/ATP exchange between the mitochondrion and
the cytosol that keeps the cell’s ATP pool highly charged. The resulting high cellular
concentration of ATP makes the free-energy change for ATP hydrolysis extremely
favorable, allowing this hydrolysis reaction to drive a very large number of energy-requiring
processes throughout the cell. The universal presence of ATP synthase
in bacteria, mitochondria, and chloroplasts testifies to the central importance of
chemiosmotic mechanisms in cells.
CHLOROPLASTS AND PHOTOSYNTHESIS
All animals and most microorganisms rely on the continual uptake of large
amounts of organic compounds from their environment. These compounds provide
both the carbon-rich building blocks for biosynthesis and the metabolic
energy for life. It is likely that the first organisms on the primitive Earth had access
to an abundance of organic compounds produced by geochemical processes, but
it is clear that these were used up billions of years ago. Since that time, virtually
all of the organic materials required by living cells have been produced by photosynthetic
organisms, including plants and photosynthetic bacteria. The core
machinery that drives all photosynthesis appears to have evolved more than 3 billion
years ago in the ancestors of present-day bacteria; today it provides the only
major solar energy storage mechanism on Earth.
The most advanced photosynthetic bacteria are the cyanobacteria, which have
minimal nutrient requirements. They use electrons from water and the energy of
sunlight to convert atmospheric CO 2 into organic compounds—a process called
carbon fixation. In the course of the overall reaction nH 2 O + nCO 2 → (light)
(CH 2 O) n + nO 2 , they also liberate into the atmosphere the molecular oxygen that
then powers oxidative phosphorylation. In this way, it is thought that the evolution
of cyanobacteria from more primitive photosynthetic bacteria eventually
made possible the development of the many different aerobic life-forms that populate
the Earth today.
Chloroplasts Resemble Mitochondria But Have a Separate
Thylakoid Compartment
Plants (including algae) developed much later than cyanobacteria, and their photosynthesis
occurs in a specialized intracellular organelle—the chloroplast (Figure
14–37). Chloroplasts use chemiosmotic mechanisms to carry out their energy
interconversions in much the same way that mitochondria do. Although much
larger than mitochondria, they are organized on the same principles. They have a
highly permeable outer membrane; a much less permeable inner membrane, in
which membrane transport proteins are embedded; and a narrow intermembrane
space in between. Together, these two membranes form the chloroplast envelope
(Figure 14–37D). The inner chloroplast membrane surrounds a large space called
the stroma, which is analogous to the mitochondrial matrix. The stroma contains
many metabolic enzymes and, as for the mitochondrial matrix, it is the place
where ATP is made by the head of an ATP synthase. Like the mitochondrion, the
chloroplast has its own genome and genetic system. The stroma therefore also
contains a special set of ribosomes, RNAs, and the chloroplast DNA.
An important difference between the organization of mitochondria and chloroplasts
is highlighted in Figure 14–38. The inner membrane of the chloroplast is