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.
-Based on the information in the passage, FCCP most likely causes a decrease in energy production by:

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.
-Based on the information in the passage, FCCP most likely causes a decrease in energy production by:

Quizplus · Accepted Answer

11ecba2d_0ebc_a041_a3bf_d757781bbb38_MD0008_00 The ETC produces a pH difference across the inner mitochondrial membrane as protons are pumped from the matrix to the intermembrane space. Transporting protons from high- to low-concentration areas is energetically favorable but prohibited by the membrane. Normally, the only path by which protons can cross the membrane is through ATP synthase, which can couple the released energy to ATP synthesis from ADP and inorganic phosphate (P_i). The addition of FCCP provides an alternate pathway to carry protons across the membrane from high concentration to low concentration. However, protons that enter the mitochondrial matrix by this means will not interact with ATP synthase, and therefore will not drive ATP synthesis. This phenomenon is called "decoupling." Because fewer protons are being used to drive ATP synthase, the rate of ATP synthesis will decrease. (Choice B) Carrying protons from the matrix to the intermembrane space would increase the rate of ATP synthesis, as more protons would be available to flow through ATP synthase. FCCP causes a decrease in ATP synthesis. (Choice C) FCCP will bring the proton gradient closer to equilibrium because it only carries protons down the concentration gradient. Protons can only move against the gradient (further from equilibrium) with the aid of energy input, as in the ETC. (Choice D) The passage states that the citric acid cycle is fully active in the presence of FCCP, and that reduced electron carriers are neither depleted nor accumulated. Therefore, the ETC must be able to consume the reduced electron carriers that the citric acid cycle produces, and must be fully active. Educational objective: ATP synthesis is driven by protons crossing the inner mitochondrial membrane through ATP synthase. When protons are transported across the membrane by means that circumvent ATP synthase, the energy released in the process cannot be used to produce ATP. This is known as "decoupling" because proton transfer is no longer coupled to ATP synthesis.

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