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
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Finally, mitochondria are important calcium buffers, taking up calcium from
the ER and sarcoplasmic reticulum at special membrane junctions. Cellular calcium
levels control muscle contraction (see Chapter 16) and alterations are implicated
in neurodegeneration and apoptosis. Clearly, cells and organisms depend
on mitochondria in many different ways.
We now return to the central function of the mitochondrion in respiratory ATP
generation.
A Chemiosmotic Process Couples Oxidation Energy to ATP
Production
Although the citric acid cycle that takes place in the mitochondrial matrix is considered
to be part of aerobic metabolism, it does not itself use oxygen. Only the
final step of oxidative metabolism consumes molecular oxygen (O 2 ) directly (see
Figure 14–10). Nearly all the energy available from metabolizing carbohydrates,
fats, and other foodstuffs in earlier stages is saved in the form of energy-rich compounds
that feed electrons into the respiratory chain in the inner mitochondrial
membrane. These electrons, most of which are carried by NADH, finally combine
with O 2 at the end of the respiratory chain to form water. The energy released
during the complex series of electron transfers from NADH to O 2 is harnessed in
the inner membrane to generate an electrochemical gradient that drives the conversion
of ADP + P i to ATP. For this reason, the term oxidative phosphorylation is
used to describe this final series of reactions (Figure 14–12).
The total amount of energy released by biological oxidation in the respiratory
chain is equivalent to that released by the explosive combustion of hydrogen
when it combines with oxygen in a single step to form water. But the combustion
of hydrogen in a single-step chemical reaction, which has a strongly negative ∆G,
releases this large amount of energy unproductively as heat. In the respiratory
chain, the same energetically favorable reaction H 2 + ½ O 2 → H 2 O is divided into
small steps (Figure 14–13). This stepwise process allows the cell to store nearly
half of the total energy that is released in a useful form. At each step, the electrons,
which can be thought of as having been removed from a hydrogen molecule to
NADH + ½ O 2 + H + NAD + + H 2 O
energy-conversion
processes in membrane
OXIDATIVE
PHOSPHORYLATION
ADP + P i ATP
Figure 14–12 The major net energy
conversion catalyzed by the
mitochondrion. In the process of oxidative
phosphorylation, the mitochondrial inner
membrane serves as a device that changes
one form MBoC6 of chemical-bond m14.11/14.12 energy to
another, converting a major part of the
energy of NADH oxidation into phosphatebond
energy in atp.
(A) COMBUSTION (B) BIOLOGICAL OXIDATION
H 2 ½ O 2 H 2
½ O 2
separate into H +
and electrons
+ 2 e –
EXPLOSIVE
RELEASE OF
HEAT
ENERGY
H 2 O
2H + 2H +
2 e –
much of the
energy is
harnessed and
converted to
a stored form
H 2 O
½ O 2
Figure 14–13 A comparison of biological
oxidation with combustion. (A) If
hydrogen were simply burned, nearly all of
the energy would be released in the form of
heat. (B) In biological oxidation reactions,
about half of the released energy is stored
in a form useful to the cell by means of the
electron-transport chain (the respiratory
chain) in the crista membrane of the
mitochondrion. Only the rest of the energy
is released as heat. In the cell, the protons
and electrons shown here as being derived
from H 2 are removed from hydrogen
atoms that are covalently linked to NADH
molecules.