This is the final part of the phase-II of aerobic respiration. As stated earlier, in respiration, oxidation of the substrate occurs by dehydrogenation (i.e., removal) of hydrogen atoms (2H) from the substrate. These are mostly accepted by NAD to form reduced co-enzyme NADH. In the aerobic respiration of a molecule of glucose (6-C), a total of, 10 NADH₂ are formed, (2NADH₂ in glycolysis + 8 NADH₂ in Kreb’s cycle).

At one step in Kreb’s cycle, hydrogen is accepted by FAD to form FADH₂. total, 2#FADH₂ are formed in the aerobic respiration of each glucose.

Each molecule of reduced co-enzyme thus formed in aerobic respiration (glycolysis and Kreb’s cycle) is finally oxidized by the free molecular oxygen through a process called terminal oxidation.

This involves participation of the respiratory chain, also called electron transport system (ETS).

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The respiratory chain (or the ETS) is present in the inner membrane of mitochondrion (i.e., in the cristae membrane). It consists of various enzymes and co-enzymes which act as electron carriers.

The hydrogen ions (and the electrons of hydrogen) from the oxidized substrate pass through the ETS and finally react with free molecular oxygen to form water.