Essential Cell Biology 5th edition

Mitochondria and Oxidative Phosphorylation
461
Matrix. This space contains a highly
concentrated mixture of hundreds of
enzymes, including those required for the
oxidation of pyruvate and fatty acids and
for the citric acid cycle.
Inner membrane. Folded into numerous
cristae, the inner membrane contains the
proteins that carry out oxidative
phosphorylation, including the
electron-transport chain and the ATP
synthase that makes ATP. It also contains
transport proteins that move selected
molecules into and out of the matrix.
Outer membrane. Because it contains large,
channel-forming proteins (called porins),
the outer membrane is permeable to all
molecules of 5000 daltons or less.
Intermembrane space. This space contains
several enzymes that use the ATP passing
out of the matrix to phosphorylate other
nucleotides. It also contains proteins that
are released during apoptosis (discussed
in Chapter 18).
(A)
(B)
100 nm
compartments: a large internal space called the matrix and a much narrower
intermembrane space (Figure 14–8). When purified mitochondria
are gently fractionated into separate components and their contents
analyzed (see Panel 4−3, pp. 164–165), each
ECB5
of the
e14.08-14.08
membranes, and the
spaces they enclose, are found to contain a unique collection of proteins.
The outer membrane contains many molecules of a transport protein
called porin, which forms wide, aqueous channels through the lipid bilayer
(described in Chapter 11). As a result, the outer membrane is like a sieve
that is permeable to all molecules of 5000 daltons or less, including small
proteins. This makes the intermembrane space chemically equivalent to
the cytosol with respect to the small molecules and inorganic ions it contains.
In contrast, the inner membrane, like other membranes in the cell,
is impermeable to the passage of ions and most small molecules, except
where a path is provided by the specific membrane transport proteins
that it contains. The mitochondrial matrix thus contains only those molecules
that are selectively transported into the matrix across the inner
membrane, and its contents are highly specialized.
The inner mitochondrial membrane is the site of oxidative phosphorylation,
and it is here that the proteins of the electron-transport chain and
the ATP synthase required for ATP production are concentrated. This
membrane is highly convoluted, forming a series of infoldings—known
as cristae—that project into the matrix space (see Figure 14–8 and Movie
14.3). These folds greatly increase the surface area of the membrane. In
a liver cell, the inner membranes of all the mitochondria make up about
one-third of the total membranes of the cell.
Figure 14–8 A mitochondrion is organized
into four separate compartments.
(A) A schematic drawing and (B) an
electron micrograph of a mitochondrion.
Each compartment contains a unique
set of proteins, enabling it to perform its
distinct functions. In liver mitochondria, an
estimated 67% of the total mitochondrial
protein is located in the matrix, 21% in
the inner membrane, 6% in the outer
membrane, and 6% in the intermembrane
space. (B, courtesy of Daniel S. Friend, by
permission of E.L. Bearer.)
QUESTION 14–2
Electron micrographs show that
mitochondria in heart muscle have a
much higher density of cristae than
mitochondria in skin cells. Suggest
an explanation for this observation.
The Citric Acid Cycle Generates High-Energy Electrons
Required for ATP Production
The generation of ATP is powered by the flow of electrons that are
derived from the burning of carbohydrates, fats, and other foodstuffs during
glycolysis and the citric acid cycle. These “high-energy” electrons are
provided by activated carriers generated during these two sets of catabolic
reactions, with the majority being churned out by the citric acid
cycle that operates in the mitochondrial matrix (discussed in Chapter 13).