Energy-rich molecules, such as glucose, are metabolized by a series of oxidation reactions ultimately yielding CO2 and water (Figure 6.6). The metabolic intermediates of these reactions donate electrons to specific coenzymes—nicotinamide adenine dinucleotide (NAD + ) and flavin adenine dinucleotide (FAD)—to form the energy-rich reduced coen- zymes, NADH and FADH 2 . These reduced coenzymes can, in turn, each donate a pair of electrons to a specialized set of electron carriers, collectively called the electron transport chain, described in this section. As electrons are passed down the electron transport chain, they lose much of their free energy. Part of this energy can be captured and stored by the production of ATP from ADP and inorganic phosphate (Pi). This process is called oxidative phosphorylation and is described on p. 77. The remainder of the free energy not trapped as ATP is used to drive ancillary reactions such as Ca 2+ transport into mitochondria, and to generate heat