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oxidative phosphorylation | NMDCAT 2021

About this Video! iv. Respiratory chain: In the Krebs cycle NADH and H+ are produced from NAD+ . NADH then transfers the hydrogen atom to the respiratory chain (also called electron transport system) where electrons are transported in a series of oxidation-reduction steps to react, ultimately, with molecular oxygen. The oxidation reduction substances which take part in respiratory chain are: i. A coenzyme called coenzyme Q ii. A series of cytochrome enzymes (b,c,a,a3 ) iii. Molecular oxygen (02 ) Cytochromes are electron transport intermediates containing haem of related prosthetic groups, that undergo valency changes of iron atom. Haem is the same iron containing group that is oxygen carrying pigment in haemoglobin. NADH is oxidized by coenzyme Q. This oxidation yields enough free energy to permit the synthesis of a molecule of ATP from ADP and inorganic phosphate. Coenzyme Q is in turn oxidized by cytochrome b which is then oxidized by cytochrome c. This step also yields enough energy to permit the synthesis of a molecule of ATP. Cytochrome c then reduces a complex of two enzymes called cytochrome a and as (for convenience the complex is referred as cytochrome a). Cytochrome a is oxidized by an atom of oxygen and the electrons arrive at the bottom end of the respiratory chain. Oxygen is the most electronegative substance and the inal acceptor of the electrons. A molecule of water is produced. In addition, this inal oxidation provides enough energy for the synthesis of a third molecule of ATP. Oxidative phosphorylation: Synthesis of ATP in the presence of oxygen is called oxidative phosphorylation. Normally, oxidative phosphorylation is coupled with the respiratory chain. As already described ATP is formed in three steps of die respiratory chain. The equation for this process can be expressed as follows: Where Pi is inorganic phosphate. The molecular mechanism of oxidative phosphorylation takes place in conjunction with the respiratory chain in the inner membrane of the mitochondrion. Here also, as in photosynthesis, the mechanism involved is chemiosmosis by which electron transport chain is coupled with synthesis of ATP. In this case, however the pumping/movement of protons (H+ ) is across the inner membrane of mitochondrion folded into cristae, between matrix of mitochondrion and mitochondrion’s intermembrane space. The coupling factors in respiration are also diferent from those in photosynthesis