Question

. Explain; a) What is the role of oxygen in the aerobic respiratory pathways and b) What would happen if oxygen is not present in the process.

Answer 1

Ans. #a. O₂ is the terminal electron acceptor in the aerobic respiratory pathway.

Complex IV transfers 4 electrons to each O₂ molecule which in turn associates with 4 protons from the mitochondrial matrix to form 2 H₂O molecules.

#b. If O₂ is NOT available:

I. High energy electrons from NADH and FADH2 oxidation are sequentially transferred to complex IV during ETC. The energy of these high electrons is harvested in form of proton gradient as they are transferred to complex IV.

If O₂ is NOT available, there would be no electron acceptor at complex IV. So, complex IV remains in reduced state because it can’t donate its electron to anything else. Once complex IV freezes in reduced state, it can’t accept electrons from complex III. Thus, complex III also freezes in reduced state. If further freezes complex I and II and the electron carriers in reduced state.

Therefore, the ETC freezes in reduced state and shut down. Shutting off of ETC thus prevents establishment of proton gradient and oxidative phosphorylation. Thus, no ATP will be produced in mitochondria.

II. Freezing of complex I and II also prevents oxidation of NADH and FADH2 respectively. Since the cell has limited pool of these molecules in respective compartments, unavailability of these molecules have following consequences-

            1. NADH is cycled between mitochondria and cytoplasm. Inhibition of NADH oxidation and resultant unavailability of NAD⁺ in turn inhibits glycolysis and citric acid cycle which use NAD⁺ as cofactor.

            2. Unavailability of FAD also inhibits citric acid cycle because its used as cofactor by few enzymes in the cycle.