Q: Which of the following best describes the overall stoichiometry of one turn of the citric acid cycle?

$$\text{Acetyl-CoA} + 2\text{H}_2\text{O} + 3\text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i \rightarrow 2\text{CO}_2 + 3\text{NADH} + \text{FADH}_2 + \text{ATP} + \text{CoA-SH}$$

Question 1

Which of the following statements best describes the role of the citric acid cycle in cellular metabolism?

Accepted Answer

It is the central pathway for oxidizing all metabolic fuels, storing most energy yield in reduced electron carriers such as NADH.

Question 2

During cellular respiration, which stage is responsible for the reoxidation of reduced electron carriers and the majority of ATP synthesis?

Accepted Answer

Stage 3

Question 3

Where do the reactions of stages 1 and 2 of cellular respiration occur in eukaryotic cells?

Accepted Answer

In the mitochondrial matrix

Question 4

Which enzyme complex converts pyruvate to acetyl-CoA, and what are its three main enzymatic components?

Accepted Answer

Pyruvate dehydrogenase (PDH) complex; pyruvate dehydrogenase (E1), dihydrolipoamide transacetylase (E2), dihydrolipoamide dehydrogenase (E3)

Question 5

How many reactions comprise one turn of the citric acid cycle, and what are the main products generated per turn?

Accepted Answer

Eight reactions; two CO$$_2$$, three NADH, one FADH$$_2$$

Question 6

Which reaction in the citric acid cycle is highly exergonic due to thioester hydrolysis, and why is this important?

Accepted Answer

Citrate synthase reaction; it ensures the cycle can proceed even at low concentrations of oxaloacetate.

Question 7

Which citric acid cycle enzyme catalyzes a substrate-level phosphorylation, and what is the product?

Accepted Answer

Succinyl-CoA synthetase; ATP or GTP

Question 8

What is the role of succinate dehydrogenase in the citric acid cycle and electron transport chain?

Accepted Answer

It catalyzes the dehydrogenation of succinate to fumarate and delivers electrons directly to the electron transport chain via coenzyme Q.

Question 9

Why does the malate dehydrogenase reaction proceed to the right in vivo despite its large positive standard free energy change ($$\Delta G$$^{\circ'}$$ = +29.7 kJ/$$mol)?

Accepted Answer

Because the highly exergonic citrate synthase reaction keeps oxaloacetate levels very low.

Question 10

Which of the following best describes the overall stoichiometry of one turn of the citric acid cycle?

Accepted Answer

$$	ext{Acetyl-CoA} + 2	ext{H}_2	ext{O} + 3	ext{NAD}^+ + 	ext{FAD} + 	ext{ADP} + 	ext{P}_i ightarrow 2	ext{CO}_2 + 3	ext{NADH} + 	ext{FADH}_2 + 	ext{ATP} + 	ext{CoA-SH}$$

Question 11

Which electron carrier in the respiratory chain can transfer two electrons in one step and provides a link between two-electron and one-electron carriers?

Accepted Answer

Coenzyme Q (ubiquinone)

Question 12

What is the main function of iron-sulfur clusters in the electron transport chain?

Accepted Answer

They act as single electron carriers with variable reduction potentials.

Question 13

Which process is described by the chemiosmotic coupling model proposed by Mitchell in 1961?

Accepted Answer

Electron transport drives the active transport of protons from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient.

Question 14

What provides the free energy for ATP synthesis in mitochondria according to the chemiosmotic theory?

Accepted Answer

Dissipation of the electrochemical proton gradient through ATP synthase (complex V)

Question 15

How many moles of ATP are generated per mole of glucose oxidized, considering glycolysis, citric acid cycle, and oxidative phosphorylation?

Accepted Answer

32 moles

Question 16

Which of the following is NOT a function of the mitochondrial inner membrane?

Accepted Answer

Catalyzing reactions of the citric acid cycle

Question 17

Which citric acid cycle intermediate is formed by the highly stereospecific hydration of fumarate?

Accepted Answer

L-malate

Question 18

Which reaction in the citric acid cycle is analogous to the pyruvate dehydrogenase complex and conserves energy as NADH?

Accepted Answer

α-Ketoglutarate dehydrogenase complex

Question 19

What is the significance of succinate being symmetrical in the citric acid cycle?

Accepted Answer

It means the fate of the carbon atoms from acetyl-CoA cannot be distinguished after this step.

Pyruvate Oxidation, Citric Acid Cycle, and Electron Transport: Structure, Mechanism, and Energetics

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