INTRODUCTION TO ROLE OF OXYGEN IN RESPIRATION
Oxygen acts as the final hydrogen acceptor. This is the vital role of oxygen in respiration and shows why most of the cells cannot survive without oxygen. Without oxygen, the last coenzyme of electron transmitter chain will fail to release its electron, and the entire electron transmitter chain would soon back up and no further energy would flow from food.
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A coenzyme is reduced on accepting an electron, and is oxidized on transferring to the next. The transfer of electron over the electron transmitter system is, thus, a series of oxidation reduction reaction. Oxidation and reduction are complementary. Because electron cannot float around on their own, for every oxidation there is a corresponding reduction. The oxidation reduction reactions during electron transfers over the electron transmitter system are referred to as biological oxidation.
Passage of electrons and protons of the reduced coenzyme to oxygen is called terminal oxidation. It occurs at the end of the aerobic respiration.
NADH + H ? NAD + 2H+ + 2e-
FADH2 ? FAD + 2H+ + 2e-
2H+ + 2e- ? 2H
½ O2 + 2H ? H2O
If sufficient oxygen is available, each 3-carbon pyruvate molecule molecules enters the mitochondrial matrix where its oxidation is completed by aerobic means. The pyruvate molecule gives off a molecule of carbon dioxide and release a pair of hydrogen atoms from its carboxyl group, leaving two carbon acetyl group. The reaction is called oxidative decarboxylation, and is catalyzed by the enzyme pyruvate dehydrgenase complex. During these reactions, the acetyl group combines with the coenzyme A to form acetyl coenzyme A with high energy bond. Most of the free energy released by the oxidation of pyruvate is captured as chemical energy in the high energy bond of acetyl coenzyme A.
Oxidation of food in the cells is enables the cells to store as ATP molecules a relatively high proportion of energy released by oxidation each energy-yielding step is coupled with the ATP synthesis.