Question 1

Which of the following best explains why biological oxidation in mitochondria is more efficient than direct combustion of hydrogen?

Accepted Answer

Biological oxidation stores about 50% of the released energy in a useful form for the cell, while combustion releases nearly all energy as heat.

Question 2

In the mitochondrial electron transport chain, which complex does NOT pump protons across the inner mitochondrial membrane?

Accepted Answer

Succinate dehydrogenase (Complex II)

Question 3

What is the role of ubiquinone in the electron transport chain?

Accepted Answer

It is a mobile carrier that ferries electrons from Complex I and II to Complex III.

Question 4

Which statement about iron-sulfur (Fe-S) clusters in the electron transport chain is correct?

Accepted Answer

Fe-S clusters are covalently linked to cysteines or histidines and can carry only one electron at a time.

Question 5

What happens to protons when ubiquinone (Q) is reduced to ubiquinol (QH2) in the mitochondrial membrane?

Accepted Answer

Two protons are picked up from the matrix side.

Question 6

Which of the following correctly matches the number of protons pumped per electron transferred by each respiratory enzyme complex?

Accepted Answer

NADH dehydrogenase: 4, Cytochrome c reductase: 2, Cytochrome c oxidase: 1

Question 7

Which of the following is a direct consequence of the increase in redox potential ($$E'_0) $$along the mitochondrial electron transport chain?

Accepted Answer

Electrons flow spontaneously from NADH to oxygen, releasing free energy.

Question 8

What is the function of the Rieske protein in Complex III (cytochrome c reductase)?

Accepted Answer

It contains an Fe2-S2 iron-sulfur cluster and transfers electrons within Complex III.

Question 9

Which of the following statements about the Q-cycle in Complex III is correct?

Accepted Answer

It allows for the directional release and uptake of protons, contributing to the proton gradient.

Question 10

Which subunits form the functional core of cytochrome c oxidase (Complex IV) and are encoded by the mitochondrial genome?

Accepted Answer

Subunits I, II, and III

Question 11

Cyanide (CN-) is extremely toxic because it inhibits which step in the electron transport chain?

Accepted Answer

It competes with O2 for binding to the heme iron in cytochrome c oxidase, blocking electron transfer to oxygen.

Question 12

Which equation best represents the reaction catalyzed by Complex I (NADH dehydrogenase) in the electron transport chain?

Accepted Answer

$$\mathrm{NADH} + \mathrm{H}^+ + \mathrm{Q} \rightarrow \mathrm{NAD}^+ + \mathrm{QH}_2$$

Question 13

Which of the following is a mobile electron carrier in the mitochondrial electron transport chain?

Accepted Answer

Cytochrome c

Question 14

What is the main function of the proton gradient established across the inner mitochondrial membrane?

Accepted Answer

To drive the synthesis of ATP via ATP synthase

Question 15

Which of the following best describes the structure of the heme group in cytochromes?

Accepted Answer

A porphyrin ring with a central iron atom

Question 16

If a mutation prevented the binding of ubiquinone to Complex I, what would be the most immediate effect on cellular respiration?

Accepted Answer

Electrons from NADH could not enter the electron transport chain, reducing ATP production.

16: Energy Conversion in Mitochondria: The Electron Transport Chain and Oxidative Phosphorylation

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