Oxidation of food stuff occurs in 3 stages—primary metabolisms where macromolecules are converted to smaller units, secondary metabolism where reducing equivalents are formed and tertiary metabolism where energy is released. Oxidation is loss of electrons and reduction is gain of electrons. A pair that exists in both oxidized and reduced state is a redox couple. In substrate level phosphorylation, energy from high-energy compound is directly transferred to NDP to form NTP without the help of electron transport chain. Transfer of electrons from reduced co-enzymes through respiratory chain to O2 is known as Biological Oxidation. The energy released is trapped as ATP. This coupling of oxidation with phosphorylation is called Oxidative phosphorylation. All enzymes of biological oxidation are oxidoreductases. Electron flow occurs through successive dehydrogenase enzymes (located in the inner mitochondrial membrane), together known as Electron Transport Chain; the electrons are transferred from higher to lower potential. NADH is impermeable to mitochondrial membrane. Hence it is transferred via malate—aspartate shuttle in liver, kidney and heart as NADH reducing equivalents and in skeletal muscles as FADH2 through glycerol 3-phosphate shuttle. The ETC has 4 distinct multiprotein complexes –viz; complex I, II, III and IV connected by 2 mobile carriers to CoQ and cytochrome C. Inhibitors of oxidative phosphorylation include atractyloside and oligomycin. Cyanide inhibits terminal cytochrome and brings cellular respiration to stand still.