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 CO2 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.
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 proton. For example, acetic acid (CH3COOH) is converted into its conjugate base, the acetate ion (CH3COO–), in the reaction:
In an exactly analogous way, pairs of compounds such as NADH and NAD+ are called redox pairs, since NADH is converted to NAD+ by the loss of electrons in the reaction:
NADH is a strong electron donor: because two of its electrons are engaged in a covalent bond which releases energy when broken, the free-energy change for passing these electrons to many other molecules is favorable. Energy is required to form this bond from NAD+, two electrons, and a proton (the same amount of energy that was released when the bond was broken). Therefore NAD+, the redox partner of NADH, is of necessity a weak electron acceptor.