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The mitochondrial inner membrane, cristae, and matrix are the sites of most reactions involving the oxidation of pyruvate and fatty acids to CO2 and H2O and the coupled synthesis of ATP from ADP and P(. These processes involve many steps but can be subdivided into three groups of reactions, each of which occurs in a discrete membrane or space in the mitochondrion (Figure 8-7):

1. Oxidation of pyruvate and fatty acids to CO2 coupled to reduction of NAD+ to NADH and of flavin adenine dinucleotide (FAD), another oxidized electron carrier, to its reduced form, FADH2 (see Figure 2-26). These electron carriers are often referred to as coenzymes. NAD+, NADH, FAD, and FADH2 are diffusible and not permanently bound to proteins. Most of the reactions occur in the matrix; two are catalyzed by inner-membrane enzymes that face the matrix.

2. Electron transfer from NADH and FADH2 to O2, regenerating the oxidized electron carriers NAD+ and FAD. These reactions occur in the inner membrane and are coupled to the generation of a proton-motive force across it.

3. Harnessing of the energy stored in the electrochemical proton gradient for ATP synthesis by the F0F1 complex in the inner membrane.

The cristae greatly expand the surface area of the inner mitochondrial membrane, enhancing its ability to generate ATP (see Figure 8-6). In typical liver mitochondria, for example, the area of the inner membrane including cristae is about five times that of the outer membrane. In fact, the total area of all inner mitochondrial membranes in liver cells is about i7 times that of the plasma membrane. The mitochondria in heart and skeletal muscles contain three times as many cristae as are found in typical liver mitochondria— presumably reflecting the greater demand for ATP by muscle cells.

In plants, stored carbohydrates, mostly in the form of starch, are hydrolyzed to glucose. Glycolysis then produces pyruvate, which is transported into mitochondria, as in animal cells. Mitochondrial oxidation of pyruvate and concomitant formation of ATP occur in pho-tosynthetic cells during dark periods when photosynthesis is not possible, and in roots and other nonphotosynthetic tissues all the time. I

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