Respiration (Advanced Level / Expected MCQs)

Correct Answer: (b)
In cellular respiration, the electrons in glucose are transferred to NAD⁺ to form NADH. NADH then delivers these electrons to the electron transport chain. At the end of the electron transport chain, oxygen accepts these electrons (along with protons) to form water. Choice (A) is incorrect because the final electron acceptor in cellular respiration is oxygen, not NAD⁺. In cellular respiration, pyruvate is formed prior to electrons entering the electron transport chain, so choice (C) is incorrect. Choice (D) is incorrect because carbon dioxide does not receive electrons during cellular respiration.

Correct Answer: (b)
The vast majority of ATP produced in cellular respiration is produced by chemiosmosis (approximately 34 ATPs per glucose molecule!), and glycolysis and the Krebs cycle each produce a net gain of 2 ATPs per glucose molecule. Choice (A) is incorrect because the oxidation of pyruvate does not produce any ATP. Fermentation does not produce ATP, so choices (C) and (D) are incorrect.

Correct Answer: (c)
Fermentation oxidizes NADH into NAD⁺ so that NAD⁺ can be used in glycolysis. Choice (A) is incorrect because fermentation does not produce ATP. Pyruvate is a reactant used in fermentation, not a product of fermentation, so choice (B) is incorrect. While carbon dioxide is a product of alcohol fermentation, carbon dioxide is not always produced by fermentation (for example, lactic acid fermentation does not produce carbon dioxide). So choice (D) is incorrect.

Correct Answer: (c)
The electron transport chain and ATP synthase are both located in the inner membrane of the mitochondria. Choices (A), (B), and (D) do not list the correct location and are therefore incorrect.

Correct Answer: (d)
If electrons could not travel down the electron transport chain, a proton gradient would not be produced and chemiosmosis could not occur. Choice (A) is incorrect because the production of pyruvate does not require the electron transport chain. Choice (B) is incorrect because the ETC regenerates NAD⁺, not NADH. Limited amounts of ATP are produced in glycolysis and the Krebs cycle, so choice (C) is also incorrect.

Correct Answer: (b)
During oxidative phosphorylation, NADH is oxidized to NAD⁺ and oxygen is reduced to water. Choice (A) is incorrect because oxygen is not produced in oxidative phosphorylation. Choice (C) is incorrect because, during oxidative phosphorylation, NAD⁺ is not reduced and oxygen is not oxidized (it is reduced). Choice (D) is incorrect because NAD⁺ is not reduced and oxygen is not produced during oxidative phosphorylation.

Correct Answer: (d)
Glycolysis does not produce a proton gradient; the movement of electrons down the electron transport chain produces a proton gradient. Choices (A), (B), and (C) are all true statements about glycolysis. Notice that this question was asking for the statement about glycolysis that is NOT correct, which is choice (D).

Correct Answer: (a)
Glycolysis does not produce carbon dioxide. Choices (B), (C), and (D) all produce carbon dioxide, so all three of those choices are incorrect.

Correct Answer: (c)
Chemiosmosis uses a proton gradient to drive the production of ATP. If protons were able to pass freely through the inner mitochondrial membrane, a proton gradient could not be formed and chemiosmosis could not occur. Glycolysis (choice (A)), the Krebs cycle (choice (B)), and fermentation (choice (D)) do not require a proton gradient, so those processes would not be directly affected. Thus, choices (A), (B), and (D) are not correct.

Correct Answer: (b)
Oxygen is the final (or terminal) electron acceptor in cellular respiration; oxygen combines with electrons and protons to form water. While carbon dioxide is a product of cellular respiration, oxygen does not combine with carbon during cellular respiration to produce carbon dioxide. Thus, choice (A) is incorrect. Choice (C) is incorrect because oxygen neither removes carbon from glucose nor does it form pyruvate during cellular respiration. Removing a carbon from pyruvate to form an alcohol is what happens during fermentation, not cellular respiration, so choice (D) is incorrect.