ETC

A, B, C i.e. (NADH Co-Q reductase, Cytochrome C oxidase, Co-Q-Cytochrome C reductase)

• Mitchell’s chemiosmotic hypothesis links the respiratory (i.e. electron transport) chain to ATP production (i.e. oxidative phosphorylation)Q. It explains how the free energy generated by the transport of electrons from NADH to molecular oxygen (in respiratory chain) is used to produce ATP from ADP + Fi (from proton pumps).
• For each NADH oxidized (pair of electron), complex I (NADH -Q- oxido reductase) and complex III (ubiquinone /Q­cytochrome C- oxidoreductase) pump (translocate) 4 protons (H+) each, whereas complex IV (Cytochrome C- oxidase) translocates only 2 protons (H+). In ETC. oxidative phosphorylation (ATP formation) occurs at these sites (ie complex I, III & IV)

Ans. is ‘c’ i.e., To act as last electron acceptor

Structural organizations of components of ETC

3 Components of respiratory chain do not function as discrete carriers of reducing equivalent but are organized into four complexes each of which acts as a specific oxidoreductase. Coenzyme Q and cytochrome C are not parts of any complex and are not fixed in the inner mitochondria! membrane. The other components are fixed in the membrane. These components are arranged in order of increasing redox potential. Therefore, reducing equivalents (electrons) flow in one direction, I —> II –> III –> IV, only because redox couple with low redox potential is better electron donor where as the one with high redox potential is electron acceptor. Thus, reducing equivalents (electrons) flow through the chain from the components of more negative redox potential to the components of more positive redox potential.

i) Complex I (NADH – CoQ reductase) catalyzes the transfer of electron from NADH to coenzyme Q (CoQ).

ii) Complex II (Succinate – CoQ reductase or succinate dehydrogenase) transfers electrons from succinate to coenzyme Q.

iii) Complex III (CoQ – cytochrome C reductase), transfers electron from CoQ to cytochrome C.

iv) Complex IV (cytochrome C oxidase) transfers electrons from cytochrome C to 02.

Ans. is ‘b’ i.e., Complex II

• During the transfer of electrons through the ETC, energy is produced. This energy is coupled to the formation of ATP molecules by phosphorylation of ADP by an enzyme F₀F₁ ATPase. The phosphorylation of ADP into ATP is coupled with oxidation of reducing equivalents, therefore the process is called oxidative phosphorylation.
• There are three ATP synthesizing sites of electron transport chain : (i) Site 1 is between NAD and CoQ, i.e., Complex I; (ii) Site II is between CoQ and cytochrome C, i.e., Complex III; and (iii) Site III is between cytochrome C and oxygen, i.e., complex IV. These sites provide energy required to make ATP from ADP by an enzyme FoFiATPase.
• The energy liberated of site I (complex I) is used to synthesize 1 ATP molecule, at site II (complex III) is used to synthesize 1 ATP molecule and at site III (Complex IV) is used to synthesize 1/2 ATP molecule. Thus, when 1 NADH molecule enters the respiratory chain, it produce 2.5 molecules of ATP. When 1 molecule of FADH2 enters the respiratory chain only 1.5 molecules of ATP are produced as site I of energy liberation is bypassed. Note : Previously it was assumed the NADH produces 3 ATPs and FAD generates 2 ATPs. Recent experiments show that these old values are overestimates and NADH produces 2.5 ATPs and FADH2 produces 1.5 ATPs.

Ans. is ‘c’ i.e., No role of inorganic phosphate

Ans. is ‘b’ i.e., Inhibition of ATP synthesis only not ETC

As the name suggests, these componds block the coupeling of oxidation with phosphorylation.

These compounds allow the transfer of reducing equivalents in respiratory chain but prevent the phosphorylation of ADP to ATP by uncoupling the linkage between ETC and phosphorylation.

Thus the energy instead of being trapped by phosphorylation is dissipated as heat. Uncouplers may be :‑

i) Natural :- Thermogenin, thyroxine

ii) Synthetic :- 2, 4-dinitrophenol (2, 4-DNP), 2, 4-dinitrocresol (2, 4-DNC), and CCCP (chlorocarbonylcyanidephenyl hydrazone).

Ans. is `d’ i.e., IV