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 PROTON PUMPS OF THE ELECTRON-TRANSPOrt CHAIN
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The electrochemical gradient across the inner membrane of a respiring mitochondrion
is typically about 180 mV (inside negative), and it consists of a membrane
potential of about 150 mV and a pH gradient of about 0.5 to 0.6 pH units
(each ∆pH of 1 pH unit is equivalent to a membrane potential of about 60 mV).
The electrochemical gradient drives not only ATP synthesis but also the transport
of selected molecules across the inner mitochondrial membrane, including the
import of selected proteins from the cytoplasm (discussed in Chapter 12).
Summary
The mitochondrion performs most cellular oxidations and produces the bulk of the
animal cell’s ATP. A mitochondrion has two separate membranes: the outer membrane
and the inner membrane. The inner membrane surrounds the innermost
space (the matrix) of the mitochondrion and forms the cristae, which project into
the matrix. The matrix and the inner membrane cristae are the major working parts
of the mitochondrion. The membranes that form cristae account for a major part
of the membrane surface area in most cells, and they contain the mitochondrion’s
electron-transport chain (the respiratory chain).
The mitochondrial matrix contains a large variety of enzymes, including those
that convert pyruvate and fatty acids to acetyl CoA and those that oxidize this acetyl
CoA to CO 2 through the citric acid cycle. These oxidation reactions produce large
amounts of NADH, whose high-energy electrons are passed to the respiratory chain.
The respiratory chain then uses the energy derived from transporting electrons from
NADH to molecular oxygen to pump H + out of the matrix. This produces a large
electrochemical proton gradient across the inner mitochondrial membrane, composed
of contributions from both a membrane potential and a pH difference. This
electrochemical gradient exerts a force to drive H + back into the matrix. This proton-motive
force is harnessed both to produce ATP and for the selective transport of
metabolites across the inner mitochondrial membrane.
THE PROTON PUMPS OF THE ELECTRON-
TRANSPOrt CHAIN
Having considered in general terms how a mitochondrion uses electron transport
to generate a proton-motive force, we now turn to the molecular mechanisms
that underlie this membrane-based energy-conversion process. In describing the
respiratory chain of mitochondria, we accomplish the larger purpose of explaining
how an electron-transport process can pump protons across a membrane. As
stated at the beginning of this chapter, mitochondria, chloroplasts, archaea, and
bacteria use very similar chemiosmotic mechanisms. In fact, these mechanisms
underlie the function of all living organisms—including anaerobes that derive all
their energy from electron transfers between two inorganic molecules, as we shall
see later.
We start with some of the basic principles on which all of these processes
depend.
The Redox Potential Is a Measure of Electron Affinities
In chemical reactions, any electrons removed from one molecule are always
passed to another, so that whenever one molecule is oxidized, another is reduced.
As with any other chemical reaction, the tendency of such redox reactions to
proceed spontaneously depends on the free-energy change (∆G) for the electron
transfer, which in turn depends on the relative affinities of the two molecules for
electrons.
Because electron transfers provide most of the energy for life, it is worth taking
the time to understand them. As discussed in Chapter 2, acids donate protons
and bases accept them (see Panel 2–2, p. 93). Acids and bases exist in conjugate
acid–base pairs, in which the acid is readily converted into the base by the loss of a