AP Biology: Pyruvate Oxidation and the Citric Acid Cycle Practice Questions
Follow carbon and electrons from pyruvate into the citric acid cycle. These original questions use simplified cell experiments and pathway accounting to connect the reactions to aerobic respiration. This is independent study practice, not an official AP resource.
Free original practice questions for study purposes. Open Exam Prep is an independent study resource and is not affiliated with, endorsed by, or sponsored by the makers of AP Exams.
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Answer the questions below — you get instant feedback and a full explanation for each.
1. A eukaryotic cell has transported pyruvate into a mitochondrion. In which compartment does the pyruvate dehydrogenase complex convert it to acetyl-CoA?
Explanation. Pyruvate dehydrogenase acts in the mitochondrial matrix. Transport into this compartment connects cytosolic glycolysis with acetyl-CoA formation and the subsequent citric acid cycle.
2. In an experiment, exactly five pyruvate molecules undergo pyruvate oxidation. How many acetyl-CoA molecules and carbon dioxide molecules does this step produce?
Explanation. Each three-carbon pyruvate loses one carbon as carbon dioxide and supplies one two-carbon acetyl group to CoA. Multiplying this one-to-one relationship by five gives five of each product.
3. An enzyme preparation converts pyruvate to acetyl-CoA while reducing a carrier. Which change identifies that carrier's role?
Explanation. Oxidation of the carbon substrate is coupled to reduction of NAD+ to NADH. NADH retains the transferred electrons for later use in respiration; it is not the carbon carrier or a direct phosphate donor here.
4. An isolated pyruvate dehydrogenase preparation converts two pyruvate molecules to two acetyl-CoA molecules. No other pathway is present. How much ATP is made directly by this conversion?
Explanation. Pyruvate oxidation produces NADH and acetyl-CoA but does not directly phosphorylate ADP. ATP subsequently supported by NADH requires additional machinery excluded from this preparation.
5. At the first reaction of the citric acid cycle, the acetyl group entering on CoA combines with oxaloacetate. Which carbon count is consistent with citrate formation?
Explanation. The incoming acetyl group contributes two carbons, and oxaloacetate contributes four. Their combination produces six-carbon citrate. The CoA molecule separates rather than becoming part of citrate's carbon skeleton.
6. A model counts six complete turns of the standard citric acid cycle, excluding pyruvate oxidation. How many NADH and FADH2 are formed?
Explanation. Each cycle turn forms three NADH and one FADH2. Six turns therefore form eighteen NADH and six FADH2. NADH made before cycle entry during pyruvate oxidation is outside the stated boundary.
7. A cell sends the two pyruvate molecules obtained from one glucose through pyruvate oxidation and then completes two citric acid cycle turns. How many carbon dioxide molecules are released across these steps?
Explanation. The two pyruvate oxidation reactions release two carbon dioxide molecules. Two cycle turns release four more, making six overall. This counts total releases; it does not assert that every released carbon came from the newly supplied glucose on those first turns.
8. A researcher removes oxaloacetate from a cycle preparation and prevents any replacement. Acetyl-CoA remains abundant. Why does citrate formation decline?
Explanation. Citrate formation requires both an acetyl group and oxaloacetate. The cycle normally regenerates oxaloacetate, but removing it without replenishment limits entry even when acetyl-CoA is available.
9. During the conversion of succinyl-CoA to succinate, a preparation forms GTP directly. Which energy-transfer mechanism does this illustrate?
Explanation. Energy from an organic reaction drives nucleotide phosphorylation directly, which is substrate-level phosphorylation. This step can produce GTP or ATP depending on the enzyme form; it does not require ATP synthase to use a proton gradient.
10. In a cell relying on aerobic electron transport, oxygen becomes unavailable and no alternative route reoxidizes the reduced carriers. Why will citric acid cycle flux eventually decrease?
Explanation. The cycle does not consume oxygen directly. Aerobic electron transport uses oxygen and supports regeneration of oxidized carriers. Without that regeneration or an alternative route, oxidation reactions become constrained.
11. A student follows the acetyl group entering a cycle turn and sees that two carbon dioxide molecules are released during that turn. Which conclusion about carbon identity is justified?
Explanation. Total carbon accounting and the identity of individual atoms are different questions. Carbon from the incoming acetyl group can remain in cycle intermediates and be released in later turns, so two releases do not prove immediate loss of those same two atoms.
12. In a cell supplied only with glucose as an energy source, pyruvate dehydrogenase activity falls sharply. Other enzymes initially remain active, and alternative sources of acetyl-CoA are absent. Which change most directly links this inhibition to the citric acid cycle?
Explanation. Pyruvate dehydrogenase supplies acetyl-CoA from glucose-derived pyruvate. Inhibiting this link reduces that supply to citrate formation. The stated absence of alternative acetyl-CoA sources prevents other fuels from bypassing the inhibited step.
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FAQ
How should I separate pyruvate oxidation from the citric acid cycle?
Track them as distinct steps: pyruvate oxidation prepares the acetyl group for entry, while the cycle combines that group with an acceptor and regenerates the acceptor. Keep their product counts separate until the question asks for a combined total.
Are Krebs cycle, TCA cycle, and citric acid cycle different pathways?
They are names for the same pathway. When a question uses any of these names, apply the same cycle accounting and distinguish one turn from the two turns associated with the two pyruvate molecules derived from one glucose.
Does one correct carbon total prove where a labeled carbon atom ends up?
No. A total measures how many carbons enter or leave; tracing a particular label requires following that atom through the reactions. Do not assume that a carbon released in a turn must have entered in that same turn.