Figure 14–11 The structure of cardiolipin. Cardiolipin consists of two covalently linked phospholipid units, with a total of four rather than the usual two fatty acid chains (see Figure 10–3). Cardiolipin is only produced in the mitochondrial inner membrane, where it interacts closely with membrane proteins involved in oxidative phosphorylation and atp transport. In cristae, its two juxtaposed phosphate groups may act as a local proton trap on the membrane surface.
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 (O2) 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 O2 at the end of the respiratory chain to form water. The energy released during the complex series of electron transfers from NADH to O2 is harnessed in the inner membrane to generate an electrochemical gradient that drives the conversion of ADP + Pi 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 H2 + ½ O2 →H2O 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
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 of chemical-bond energy to another, converting a major part of the energy of NaDh oxidation into phosphate-bond energy in atp.
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 h2 are removed from hydrogen atoms that are covalently linked to NaDh molecules.
produce two protons, pass through a series of electron carriers in the inner mitochondrial membrane. At each of three distinct steps along the way (marked by the three electron-transport complexes of the respiratory chain, see below), much of the energy is utilized for pumping protons across the membrane. At the end of the electron-transport chain, the electrons and protons recombine with molecular oxygen into water.