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 molecules is a precursor to the citric acid cycle?

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 molecules is a precursor to the citric acid cycle?

Quizplus · Accepted Answer

11ecba2d_0eb9_e11e_a3bf_77cd829763c4_MD0008_00 Pyruvate (shown in choice B) is a three-carbon molecule produced in the final step of glycolysis, which takes place in the cytosol. After pyruvate is synthesized, it is transported to the mitochondria, where it is decarboxylated to form acetyl-CoA. Acetyl-CoA then enters the citric acid cycle. The other molecules shown are intermediates of the citric acid cycle, not precursors. With the exceptions of acetyl-CoA and succinyl-CoA, molecules in the citric acid cycle can be easily identified by the fact that they are all flanked by carboxylate ions on both ends of the main carbon chain. Succinyl-CoA and acetyl-CoA can be identified by their thioesters (S-CoA). Of the choices given, pyruvate is the only molecule with a single carboxylate and no thioesters. (Choice A) This molecule is citrate, a six-carbon molecule that is synthesized in the first step of the citric acid cycle when acetyl-CoA combines with oxaloacetate. (Choice C) This molecule is oxaloacetate, a four-carbon molecule that forms in the last step of the citric acid cycle when malate is oxidized. (Choice D) This molecule is α-ketoglutarate, a five-carbon molecule that forms in the citric acid cycle when isocitrate releases CO₂. Educational objective: Glycolysis occurs in the cytosol and ends with the synthesis of pyruvate. Pyruvate is transported into the mitochondria, where it is degraded to acetyl-CoA and enters the citric acid cycle. All steps of the citric acid cycle occur in the mitochondria and, unlike glycolysis, involve thioesters and multiple carboxylate groups.

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