Biological oxidation involves the loss of electrons and/or hydrogen atoms from a substrate. This process is carried out by enzymes and can involve the loss of electrons, hydrogen atoms, or addition of oxygen atoms. Energy released from exergonic reactions is transferred through common intermediates to drive endergonic reactions. Adenosine triphosphate (ATP) is often used as an energy carrier in coupled reactions, transferring phosphate groups from energy-rich intermediates to adenosine diphosphate (ADP) to form ATP. During metabolism, electrons from metabolic intermediates are transferred to electron carriers like NADH and FADH2 in the electron transport chain located in the inner mitochondrial membrane. As electrons are passed