The aim of this chapter is to focus on the TCA cycle and biological oxidation. Sequences of reactions in mitochondria that oxidize the acetyl moiety of acetyl-CoA and reduces coenzymes which are reoxidized through the electron transport chain, linked to the formation of ATP. The citric acid cycle is the final common pathway for the oxidation of carbohydrate, lipid, and protein. This chapter also covers the shuttle mechanisms, inhibitors of electron transport system, high energy compounds, electron transport chain, and inhibitors of electron transport chain. NADH cannot penetrate the mitochondrial membrane, but it is produced continuously in the cytosol by 3-phosphoglyceraldehyde dehydrogenase, an enzyme in the glycolysis sequence. The transfer of reducing equivalents is carried out by using the various shuttle systems. The flow of electrons through the respiratory chain generates ATP by the process of oxidative phosphorylation.