Question 1

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

Accepted Answer

It is the central pathway for oxidizing metabolic fuels and generating reduced electron carriers.

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: Electron transport and oxidative phosphorylation

Question 3

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

Accepted Answer

In the mitochondrial matrix

Question 4

Which enzyme complex is responsible for converting pyruvate to acetyl-CoA, and where is it located?

Accepted Answer

Pyruvate dehydrogenase complex in the mitochondrial matrix

Question 5

What is the main fate of the carbon atoms from acetyl-CoA in the citric acid cycle?

Accepted Answer

They are released as CO$_2$ during the cycle.

Question 6

Which reaction in the citric acid cycle is highly exergonic due to the hydrolysis of a thioester bond, ensuring the cycle proceeds even at low oxaloacetate concentrations?

Accepted Answer

Citrate synthase reaction

Question 7

Which of the following is a substrate-level phosphorylation reaction in the citric acid cycle?

Accepted Answer

Succinyl-CoA synthetase catalyzing the conversion of succinyl-CoA to succinate

Question 8

Which electron carrier is directly reduced during the succinate dehydrogenase reaction in the citric acid cycle?

Accepted Answer

FAD

Question 9

Why does the malate dehydrogenase reaction proceed in vivo despite its large positive standard free energy change ($	riangle G^	ext{0'}$)?

Accepted Answer

Because oxaloacetate and NADH concentrations are kept very low by subsequent reactions.

Question 10

How many NADH molecules are produced per turn of the citric acid cycle?

Accepted Answer

Three

Question 11

Which of the following best explains the chemiosmotic coupling model for ATP synthesis?

Accepted Answer

Electron transport drives proton transport, creating an electrochemical gradient that powers ATP synthase.

Question 12

What is the total ATP yield per mole of glucose oxidized, considering glycolysis, citric acid cycle, and oxidative phosphorylation?

Accepted Answer

32 moles

Question 13

Which of the following electron carriers in the mitochondrial respiratory chain can transfer two electrons in one step and acts as a link between two-electron and one-electron carriers?

Accepted Answer

Coenzyme Q (ubiquinone)

Question 14

Which component of the mitochondrial electron transport chain contains heme groups and is classified based on its absorption spectra?

Accepted Answer

Cytochromes

Question 15

A patient presents with a genetic defect in the pyruvate dehydrogenase complex. Which metabolic consequence is most likely?

Accepted Answer

Impaired conversion of pyruvate to acetyl-CoA, leading to lactic acidosis

Question 16

Which of the following reactions in the citric acid cycle involves the release of CO$_2$ and the reduction of NAD$^+$?

Accepted Answer

Isocitrate dehydrogenase converting isocitrate to $	ext{α}$-ketoglutarate

Question 17

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

Accepted Answer

Mitochondrial matrix

Question 18

If a toxin inhibits the electron transport chain, which immediate effect would you expect on the citric acid cycle?

Accepted Answer

Accumulation of NADH and decreased activity of the citric acid cycle

Question 19

Which of the following best describes the function of iron-sulfur clusters in the mitochondrial respiratory chain?

Accepted Answer

They facilitate electron transfer due to variable reduction potentials.

Question 20

Which equation correctly represents the overall reaction for one turn of the citric acid cycle?

Accepted Answer

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

Pyruvate Oxidation and the Citric Acid Cycle: Structure, Function, and Bioenergetics

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