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

THE MITOCHONDRION
759
MATRIX
FOOD MOLECULES FROM CYTOSOL
fatty acids
pyruvate
CRISTA
SPACE
citric
acid
CO 2
cycle
NADH NAD +
O 2 O 2
IN
H 2 O
e –
H +
fatty acids
H +
acetyl CoA
H +
pyruvate
ATP synthase
H +
ADP
ATP
CO 2
OUT
ADP
IN
ATP
OUT
Figure 14–10 A summary of the energyconverting
metabolism in mitochondria.
Pyruvate and fatty acids enter the
mitochondrion (top of the figure) and are
broken down to acetyl CoA. The acetyl
CoA is metabolized by the citric acid cycle,
which reduces NAD + to NADH, which then
passes its high-energy electrons to the first
complex in the electron-transport chain. In
the process of oxidative phosphorylation,
these electrons pass along the electrontransport
chain in the inner membrane
cristae to oxygen (O 2 ). This electron
transport generates a proton gradient,
which drives the production of atp by the
ATP synthase (see Figure 14–3). Electrons
from the oxidation of succinate, a reaction
intermediate in the citric acid cycle (see
Panel 2–9, pp. 106–107), take a separate
path to enter this electron-transport chain
(not shown, see p. 772).
The membranes that comprise the
mitochondrial inner membrane—the
inner boundary membrane and the crista
membrane—contain different mixtures of
proteins and they are therefore shaded
differently in this diagram.
INTERMEMBRANE SPACE
inner mitochondrial membrane
outer mitochondrial membrane
The acetyl groups in acetyl CoA are oxidized in the matrix via the citric acid
cycle, also called the Krebs MBoC6 cycle n14.306/14.10
(see Figure 2–57 and Movie 2.6). The oxidation of
these carbon atoms in acetyl CoA produces CO 2 , which diffuses out of the mitochondrion
to be released to the environment as a waste product. More importantly,
the citric acid cycle saves a great deal of the bond energy released by this
oxidation in the form of electrons carried by NADH. This NADH transfers its electrons
from the matrix to the electron-transport chain in the inner mitochondrial
membrane, where—through the chemiosmotic coupling process described previously
(see Figures 14–2 and 14–3)—the energy that was carried by NADH electrons
is converted into phosphate-bond energy in ATP. Figure 14–10 outlines this
sequence of reactions schematically.
The matrix contains the genetic system of the mitochondrion, including the
mitochondrial DNA and the ribosomes. The mitochondrial DNA (see section on
genetic systems, p. 800) is organized into compact bodies—the nucleoids—by
special scaffolding proteins that also function as transcription regulatory proteins.
The large number of enzymes required for the maintenance of the mitochondrial
genetic system, as well as for many other essential reactions to be outlined next,
accounts for the very high protein concentration in the matrix; at more than 500
mg/mL, this concentration is close to that in a protein crystal.
Mitochondria Have Many Essential Roles in Cellular Metabolism
Mitochondria not only generate most of the cell’s ATP; they also provide many
other essential resources for biosynthesis and cell growth. Before describing in
detail the remarkable machinery of the respiratory chain, we diverge briefly to
touch on some of these important roles.