Bioenergetics is the study of the energy changes accompanying biochemical reactions. ATP is the chemical link between catabolism and anabolism. ATP acts as a energy currency of the cell, transferring free energy derived from substances of high energy potential to those of the lower energy potential. Enzymes involved in biological oxidation and reduction are of the class oxidoreductases. Electron transport or respiratory chain is a series of highly organized oxidation-reduction enzymes, coenzymes and cytochromes in the inner mitochondrial membrane. The energy released from electron transport is used for the oxidative phosphorylation of ADP to ATP. According to chemiosmosis hypothesis proton gradient provide energy (proton-motive force) for ATP synthesis from ADP and Pi with the help of F0F1 ATPase. Formation of ATP can be inhibited by inhibiting electron transport or by inhibiting oxidative phosphorylation or by uncoupling oxidative phosphorylation by various compounds. Glycerol phosphate shuttle and malate aspartate shuttle are the systems for oxidation of extra mitochondrial NADH. Disorders involving components of oxidative phosphorylation referred to as OXPHOS.