Citric Acid Cycle Quiz
Questions: 16 · 10 minutes
1. How many carbon dioxide molecules are produced directly during one turn of the cycle per acetyl-CoA oxidized?
Two
One
Three
None
2. A cell has abundant ATP and a high NADH-to-NAD+ ratio. What effect is most likely on the citric acid cycle?
The cycle accelerates to generate additional reduced carriers
Citrate synthase reverses and releases acetyl-CoA
Key regulatory enzymes are inhibited, slowing cycle flux
Every cycle enzyme becomes inactive at the same moment
3. What is the net production of reduced electron carriers during one turn per acetyl-CoA?
One NADH and three FADH2
Two NADH and two FADH2
Three NADH and one FADH2
Four NADH and no FADH2
4. At the start of the cycle, acetyl-CoA contributes an acetyl group that combines directly with which molecule?
Pyruvate
Alpha-ketoglutarate
Oxaloacetate
Succinate
5. A severe thiamine deficiency reduces the activity of enzymes requiring thiamine pyrophosphate. Which citric acid cycle enzyme is directly affected?
Malate dehydrogenase
Fumarase
Succinate dehydrogenase
Alpha-ketoglutarate dehydrogenase
6. Why is the citric acid cycle described as an amphibolic pathway?
It functions only when both glucose and fatty acids are present
It alternates between the cytosol and mitochondrial matrix
It can run fully forward or fully backward under normal human physiology
It supports fuel oxidation while also supplying intermediates for biosynthesis
7. In a typical eukaryotic cell, where do most reactions of the citric acid cycle occur?
The mitochondrial matrix
The cytosol
The mitochondrial intermembrane space
The inner mitochondrial membrane
8. A compound blocks isocitrate dehydrogenase. Which immediate pattern is most likely?
Succinate accumulates while fumarate production falls
Isocitrate accumulates while alpha-ketoglutarate production falls
Malate accumulates while citrate production rises
Oxaloacetate accumulates while acetyl-CoA production stops
9. Which reaction is coupled to substrate-level phosphorylation that can produce GTP or ATP?
Citrate to isocitrate
Malate to oxaloacetate
Fumarate to malate
Succinyl-CoA to succinate
10. During which conversion are electrons transferred to FAD to form FADH2?
Isocitrate to alpha-ketoglutarate
Succinate to fumarate
Alpha-ketoglutarate to succinyl-CoA
Malate to oxaloacetate
11. Which citric acid cycle enzyme is embedded in the inner mitochondrial membrane and also functions as part of the electron transport chain?
Succinate dehydrogenase
Citrate synthase
Aconitase
Isocitrate dehydrogenase
12. A student claims oxaloacetate must be supplied in equal amounts with acetyl-CoA because it is consumed once per turn. What best corrects this claim?
Oxaloacetate participates in citrate formation but is regenerated by the end of the cycle
Oxaloacetate is converted permanently into acetyl-CoA after citrate forms
Oxaloacetate is unnecessary because acetyl-CoA enters the cycle by itself
Oxaloacetate supplies the two carbon atoms released during every first turn
13. A researcher supplies acetyl-CoA whose two acetyl carbons are isotope-labeled and examines the carbon dioxide released during the first turn. What is generally expected?
Both labeled carbons are immediately released as carbon dioxide
Exactly one labeled carbon is released before citrate forms
Most labeled acetyl carbons remain in cycle intermediates during the first turn
The labels are transferred directly to NADH rather than cycle intermediates
14. Fluorocitrate inhibits aconitase in an experimental cell preparation. Which change would be most directly expected?
Rapid conversion of citrate into alpha-ketoglutarate
Citrate accumulation and reduced isocitrate formation
Succinate accumulation and increased FADH2 production
Faster regeneration of oxaloacetate from malate
15. Oxygen is absent, so the electron transport chain can no longer oxidize NADH efficiently. Why does the citric acid cycle slow?
Oxygen is a direct substrate for citrate synthase
NADH accumulates and the supply of NAD+ becomes limited
Acetyl-CoA can no longer contain carbon atoms
Fumarase begins converting fumarate directly into pyruvate
16. Oxaloacetate is being withdrawn from the cycle for biosynthesis. Which reaction can directly replenish it in many animal tissues?
Acetyl-CoA carboxylation to malonyl-CoA
Citrate cleavage to acetyl-CoA and oxaloacetate in the cytosol
Lactate oxidation to pyruvate
Pyruvate carboxylation to oxaloacetate