Q: What is the overall balanced equation for one turn of the citric acid cycle?

$$\text{Acetyl-CoA} + 2H_2O + 3NAD^+ + FAD + ADP + P_i \rightarrow 2CO_2 + 3NADH + $$FADH_2$$ + ATP + CoA$$

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, with most energy stored 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 component enzymes?

Accepted Answer

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

Question 5

What is the net result of one turn of the citric acid cycle in terms of high-energy molecules produced?

Accepted Answer

One ATP (or GTP), three NADH, and one FADH2

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; it ensures the cycle can proceed even at low oxaloacetate concentrations

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

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 9

Which electron carrier in the mitochondrial 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 10

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 11

Which citric acid cycle enzyme is membrane-bound and delivers electrons directly into the mitochondrial electron transport system via coenzyme Q?

Accepted Answer

Succinate dehydrogenase

Question 12

What is the overall balanced equation for one turn of the citric acid cycle?

Accepted Answer

$$	ext{Acetyl-CoA} + 2H_2O + 3NAD^+ + FAD + ADP + P_i ightarrow 2CO_2 + 3NADH + $$FADH_2$$ + ATP + CoA$$

Question 13

Which process provides most of the energy used for ATP synthesis in eukaryotic cells?

Accepted Answer

Reoxidation of NADH and FADH2 in the electron transport chain

Question 14

What is 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 15

How many moles of ATP are generated per mole of glucose oxidized, including oxidative phosphorylation, and what is the calculation based on P/O ratios?

Accepted Answer

32 moles; $$4 + (2.5 	imes 10) + (1.5 	imes 2) = 32$$

Question 16

Which of the following is true about the symmetry of succinate in the citric acid cycle?

Accepted Answer

Succinate is symmetrical, making it impossible to track the fate of individual carbons after its formation.

Question 17

Which citric acid cycle enzyme is highly stereospecific, attacking only the trans double bond of fumarate to form L-malate?

Accepted Answer

Fumarase

Question 18

What is the main reason that the electron transport chain uses a variety of electron carriers?

Accepted Answer

To allow for the transfer of electrons from low to high reduction potential carriers

Question 19

Which mitochondrial compartment contains the enzymes for the citric acid cycle?

Accepted Answer

Mitochondrial matrix

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

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