Passage
Catabolism is an oxidative process in which electrons are transferred from lipids, proteins, and carbohydrates to generate high-energy electron carriers from nicotinamide adenine dinucleotide (NADH/NAD⁺) and flavin adenine dinucleotide (FADH₂/FAD) . Both are used in the electron transport chain (ETC) as electron providers in a series of reactions. The transfer of electrons provides energy that drives oxidative phosphorylation and the production of ATP. Reduction of NAD⁺ proceeds according to the following half-reaction:
Reaction 1Reactions in the ETC occur in four protein complexes (I-IV) located in the inner mitochondrial membrane. Here high-energy electrons are shuttled from complexes I, II, and III to complex IV, where O₂ serves as the final electron acceptor. The net result of NADH oxidation is described by Reaction 2.
NADH+H++12O2→NAD++H2OReaction 2At complexes I, III, and IV, hydrogen ions move from the mitochondrial matrix to the intermembrane space and create the electrochemical gradient, or proton motive force (pmf) , that drives ATP synthesis. Each NADH molecule drives the net pumping of 10 protons across the membrane, as per Reaction 3.
NADH+11H++12O2→NAD++10H++H2OReaction 3
Adapted from Garrett R, Grisham CM, Sabat M. Biochemistry. Cengage Learning; 2011.
-If the ETC is a spontaneous process, which of the following MUST be true about the redox reactions within each of the four complexes?

Question

Passage
Catabolism is an oxidative process in which electrons are transferred from lipids, proteins, and carbohydrates to generate high-energy electron carriers from nicotinamide adenine dinucleotide (NADH/NAD⁺) and flavin adenine dinucleotide (FADH₂/FAD) . Both are used in the electron transport chain (ETC) as electron providers in a series of reactions. The transfer of electrons provides energy that drives oxidative phosphorylation and the production of ATP. Reduction of NAD⁺ proceeds according to the following half-reaction:

Reaction 1Reactions in the ETC occur in four protein complexes (I-IV) located in the inner mitochondrial membrane. Here high-energy electrons are shuttled from complexes I, II, and III to complex IV, where O₂ serves as the final electron acceptor. The net result of NADH oxidation is described by Reaction 2.

NADH+H++12O2→NAD++H2OReaction 2At complexes I, III, and IV, hydrogen ions move from the mitochondrial matrix to the intermembrane space and create the electrochemical gradient, or proton motive force (pmf) , that drives ATP synthesis. Each NADH molecule drives the net pumping of 10 protons across the membrane, as per Reaction 3.

NADH+11H++12O2→NAD++10H++H2OReaction 3
Adapted from Garrett R, Grisham CM, Sabat M. Biochemistry. Cengage Learning; 2011.
-If the ETC is a spontaneous process, which of the following MUST be true about the redox reactions within each of the four complexes?

Quizplus · Accepted Answer

11ecba2d_119e_d7da_a3bf_4f18f024c286_MD0008_00 Standard reduction potentials are indicative of whether a molecule will spontaneously gain or lose electrons. In a spontaneous process, ΔE₀′ is positive. The ETC is a series of spontaneous processes, so all of the standard reduction potentials must fall within the range of NAD⁺ (E₀′ = −0.320 V) and O₂ (E₀′ = 0.816 V), and each step in the chain must have a larger ΔE₀′ than the previous step. (Choice A) The reduction potentials for each electron carrier must be less than that of oxygen for the process to be spontaneous. (Choice B) For a process to be spontaneous, the Gibbs free energy ΔG°′ must be negative. Gibbs free energy is inversely proportional to standard reduction potential, as described by the Nernst equation, ΔG°′ = −nF ΔE₀′. (Choice D) The reactions in the ETC move spontaneously from one complex to the next. However, the spontaneity of a reaction does not provide a direct correlation to the rate of a reaction. Educational objective: Standard reduction potentials (ΔE₀′) indicate whether a molecule will spontaneously gain or lose electrons, and are related to the Gibbs free energy (ΔG°′). In a spontaneous process, ΔE₀′ is positive, ΔG°′ is negative, and electrons flow from molecules with low reduction potential to molecules with high reduction potential.

Passage Catabolism Is an Oxidative Process in Which Electrons Are Transferred