Third stage of cellular respiration is the final common pathway of a cell to generate energy and occurs through the electron transport chain in which the metabolic intermediates of first and second stage of cellular respiration donate electrons to specific coenzymes – NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) to produce energy rich reduced coenzymes, NADH and FADH2. Finally, these co-enzymes, NADH and FADH2 donate a pair of electrons to a set of electron carriers to generate energy in the form of ATP.
Third stage of cellular respiration takes place in the inner mitochondrial membrane of a cell. The inner mitochondrial membrane is rich in protein and highly covulated, directly involved in electron transport system and oxidative phosphorylation. This membrane contains five septate enzyme complexes. Complexes I to IV contain electron transport system and complex V catalyzes ATP generation. Each complex donates or accepts electrons to mobile electron carriers. Each carrier in the electron transport system can receive electrons from an electron donor, and subsequently donate electrons to the next carrier in the transport system. The electrons finally combine with oxygen to form water.
Free energy is generated in this stage of cellular respiration as the electrons transferred along the electron transport system from an electron donor to an electron acceptor. The transport of one pair of electrons from NADH to oxygen through the electron transport system produces three molecules of ATP and the transport of one pair of electrons from FADH2 to oxygen through the electron transport system produces two molecules of ATP.