Passage
Mitochondria are double membrane-bound cellular organelles that house several connected metabolic reactions. The citric acid cycle takes place inside the mitochondrial matrix and produces two reduced electron carriers, NADH and FADH₂. These two carriers then pass their electrons to ubiquinone (UQ) , which is reduced to ubiquinol (UQH₂) . UQH₂ passes its electrons to oxidized cytochrome C (cyt-C_ox) , regenerating UQ and forming reduced cytochrome C (cyt-C_red) . Finally, cyt-C_red transfers electrons to oxygen, regenerating cyt-C_ox and forming water. Each reaction is facilitated by one of four protein complexes that are collectively known as the electron transport chain (ETC) .Researchers isolated mitochondria and treated them with a novel ETC inhibitor, "drug X." After an overnight incubation, they measured levels of several ETC-reduced electron carriers in the presence of drug X relative to untreated mitochondria. Their results are shown in Figure 1.
Figure 1 Relative levels of reduced electron carriers in the presence and absence of drug XNext, the researchers exposed mitochondria to FCCP, a chemical that can transport protons across membranes. They observed that in the presence of FCCP, the citric acid cycle was fully active, and all reduced cofactors were present at the same levels as in untreated mitochondria. However, the rate of ATP synthesis was significantly reduced. ATP synthesis was immediately restored when FCCP was washed away.
-Which of the following could researchers add to restore oxygen consumption in the presence of drug X?

Question

Passage
Mitochondria are double membrane-bound cellular organelles that house several connected metabolic reactions. The citric acid cycle takes place inside the mitochondrial matrix and produces two reduced electron carriers, NADH and FADH₂. These two carriers then pass their electrons to ubiquinone (UQ) , which is reduced to ubiquinol (UQH₂) . UQH₂ passes its electrons to oxidized cytochrome C (cyt-C_ox) , regenerating UQ and forming reduced cytochrome C (cyt-C_red) . Finally, cyt-C_red transfers electrons to oxygen, regenerating cyt-C_ox and forming water. Each reaction is facilitated by one of four protein complexes that are collectively known as the electron transport chain (ETC) .Researchers isolated mitochondria and treated them with a novel ETC inhibitor, "drug X." After an overnight incubation, they measured levels of several ETC-reduced electron carriers in the presence of drug X relative to untreated mitochondria. Their results are shown in Figure 1.

Figure 1 Relative levels of reduced electron carriers in the presence and absence of drug XNext, the researchers exposed mitochondria to FCCP, a chemical that can transport protons across membranes. They observed that in the presence of FCCP, the citric acid cycle was fully active, and all reduced cofactors were present at the same levels as in untreated mitochondria. However, the rate of ATP synthesis was significantly reduced. ATP synthesis was immediately restored when FCCP was washed away.
-Which of the following could researchers add to restore oxygen consumption in the presence of drug X?

Quizplus · Accepted Answer

11ecba2d_0ebb_b5e0_a3bf_614f2d1f0852_MD0008_00 Oxygen consumption takes place in complex IV together with consumption of reduced cytochrome C. ETC reactions are connected such that inhibition of one complex affects the other complexes. Figure 1 shows that reduced cytochrome C is depleted in the presence of drug X. Because complex IV can reduce oxygen only when sufficient cyt-C_red is available to donate electrons, adding more reduced cytochrome C is the only way to allow complex IV to temporarily resume oxygen consumption. (Choice B) Oxidized cytochrome C is a product of oxygen consumption. It cannot be used to reduce oxygen to water. (Choice C) Ubiquinol provides the electrons necessary for complex III to produce cyt-C_red, which in turn can reduce O₂. However, in the presence of drug X, complex III will not be able to use electrons from ubiquinol to produce cyt-C_red, no matter how much ubiquinol is added. (Choice D) Ubiquinone can be reduced to ubiquinol by complexes I and II. The generated ubiquinol would not be able to reduce cytochrome C because complex III is inactive in the presence of drug X. Educational objective: Each reduced electron carrier in the ETC facilitates reduction of the next acceptor in the chain. Depletion of one reduced electron carrier (ie, cyt-C_red), prevents reduction of the next one in the chain (ie, oxygen). Adding more of a depleted carrier will reinstate activity until it is depleted again.

Passage Mitochondria Are Double Membrane-Bound Cellular Organelles That House Several Connected