Question 1

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

Accepted Answer

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

Question 2

During which stage of cellular respiration are reduced electron carriers reoxidized to provide energy for ATP synthesis?

Accepted Answer

Stage 3

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 what are its three main enzymatic components?

Accepted Answer

Pyruvate dehydrogenase complex; E1: Pyruvate dehydrogenase, E2: Dihydrolipoamide transacetylase, E3: Dihydrolipoamide dehydrogenase

Question 5

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

Accepted Answer

Citrate synthase reaction; it ensures the cycle proceeds even at low oxaloacetate concentrations.

Question 6

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

Accepted Answer

Succinyl-CoA synthetase; succinate and ATP (or GTP)

Question 7

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

Accepted Answer

Succinate dehydrogenase

Question 8

Despite having a large positive standard free energy change ($\Delta G^{\circ'} = +29.7$ kJ/mol), the malate dehydrogenase reaction proceeds in vivo. What is the main reason for this?

Accepted Answer

The reaction is coupled to the highly exergonic citrate synthase reaction, which keeps oxaloacetate levels low.

Question 9

How many NADH and FADH$_2$ molecules are produced per turn of the citric acid cycle?

Accepted Answer

Three NADH and one FADH$_2$

Question 10

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

Accepted Answer

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

Question 11

Which electron carrier in the 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 12

What is the total ATP yield per mole of glucose oxidized, assuming P/O ratios of 2.5 for NADH and 1.5 for FADH$_2$?

Accepted Answer

$32$ ATP

Question 13

Which of the following statements about iron-sulfur clusters in the electron transport chain is correct?

Accepted Answer

They consist of nonheme iron complexed with thiol sulfurs of cysteine residues and are single electron carriers.

Question 14

A patient has a mutation that impairs the function of the pyruvate dehydrogenase complex. Which of the following metabolic consequences is most likely?

Accepted Answer

Decreased production of acetyl-CoA and reduced entry of carbon into the citric acid cycle.

Question 15

Which reaction in the citric acid cycle is analogous to the pyruvate dehydrogenase complex in terms of mechanism and cofactor requirements?

Accepted Answer

Alpha-ketoglutarate dehydrogenase complex

Question 16

Which of the following best describes the fate of the two carbon atoms from acetyl-CoA in the citric acid cycle?

Accepted Answer

They are incorporated into the cycle and released as CO$_2$ during subsequent reactions.

Question 17

Which of the following is a correct summary equation for one turn of the citric acid cycle?

Accepted Answer

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

Question 18

Which of the following statements about the localization of respiratory processes in mitochondria is correct?

Accepted Answer

Stages 1 and 2 of cellular respiration occur in the mitochondrial matrix, while stage 3 occurs at the inner mitochondrial membrane.

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

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