Figure 4.Step-by-step conversion of fuel into ATP and then ATP into biological work within the cell (30). Free fatty acids (FFAs) and glucose are oxidized generating NADH and FADH2 which donate electrons to the electron transport chain. Ubiquinone (Q) shuttles electrons from both complexes I and II to complex III while cytochrome C (C) shuttles electrons from complex III to complex IV. Molecular oxygen (O2) is the terminal electron acceptor. Protons are pumped out by complexes I, III and IV of the electron transport chain creating a proton electrochemical potential gradient (?uH+). Protons may reenter the mitochondrial matrix via the F0F1 ATPase, with energy being used to generate ATP from ADP and Pi. Protons may also reenter via an uncoupling protein (UCP), with energy being released in the form of heat. Proton rentry via ATP synthase depends upon the availability of ADP which is generated in the cytosol from reactions utilizing ATP. Abbreviations: ANC, adenine nucleotide carrier; CC, carnitine carrier; complex I, NADH-ubiquinone oxidoreductase; complex II, succinate:ubiquinone oxidoreductase; complex III, ubiquinone-cytochrome-c oxidoreductase; complex IV, cytochrome-c oxidase; PiC, phosphate carrier; PyC, pyruvate carrier. This figure was adapted from reference (29).