Krebs Cycle (TCA) Practice Questions
20 free Krebs Cycle (TCA) practice questions for the NCERT Biology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Before entering the Krebs cycle, pyruvate undergoes oxidative decarboxylation to form which 2-carbon compound?
- A Citric acid
- B Acetyl CoA
- C Oxaloacetic acid
- D Succinic acid
Correct answer: Acetyl CoA
Pyruvate produced in glycolysis enters the mitochondrial matrix and undergoes oxidative decarboxylation catalyzed by pyruvate dehydrogenase. This 'link reaction' produces Acetyl CoA, which then enters the TCA cycle.
Which enzyme complex is responsible for the conversion of Pyruvate to Acetyl CoA?
- A Pyruvate carboxylase
- B Pyruvate dehydrogenase
- C Citrate synthase
- D Succinate dehydrogenase
Correct answer: Pyruvate dehydrogenase
The pyruvate dehydrogenase complex facilitates the decarboxylation and oxidation of pyruvate. This reaction requires several coenzymes, including NAD+ and Coenzyme A.
The first step of the Krebs cycle involves the condensation of Acetyl CoA with which 4-carbon compound?
- A Malic acid
- B Succinic acid
- C Oxaloacetic acid (OAA)
- D Alpha-ketoglutaric acid
Correct answer: Oxaloacetic acid (OAA)
Acetyl CoA (2C) combines with Oxaloacetic acid (4C) to form Citric acid (6C). This reaction is catalyzed by the enzyme citrate synthase and marks the beginning of the cycle.
How many molecules of CO2 are released during one complete turn of the Krebs cycle?
- A 1
- B 2
- C 3
- D 4
Correct answer: 2
Two decarboxylation steps occur in the cycle: one during the conversion of isocitrate to alpha-ketoglutarate, and another from alpha-ketoglutarate to succinyl-CoA. This accounts for the two carbons that entered as Acetyl CoA.
At how many points in the TCA cycle is NAD+ reduced to NADH + H+?
- A One
- B Two
- C Three
- D Four
Correct answer: Three
NAD+ is reduced at three specific steps: the conversion of isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, and malate to oxaloacetate.
The conversion of succinyl-CoA to succinic acid is associated with the synthesis of:
- A ATP via oxidative phosphorylation
- B GTP or ATP via substrate-level phosphorylation
- C NADPH
- D FADH2
Correct answer: GTP or ATP via substrate-level phosphorylation
This step involves the release of CoA and the coupled synthesis of GTP from GDP (in animals) or ATP from ADP (in plants). This is the only step in the TCA cycle where a high-energy phosphate is formed directly.
Which enzyme of the Krebs cycle is uniquely located in the inner mitochondrial membrane rather than the matrix?
- A Citrate synthase
- B Alpha-ketoglutarate dehydrogenase
- C Succinate dehydrogenase
- D Malate dehydrogenase
Correct answer: Succinate dehydrogenase
Succinate dehydrogenase is membrane-bound and also acts as Complex II of the Electron Transport Chain. It catalyzes the oxidation of succinate to fumarate and the simultaneous reduction of FAD to FADH2.
FAD is reduced to FADH2 during which specific conversion in the TCA cycle?
- A Isocitrate to Alpha-ketoglutarate
- B Succinyl-CoA to Succinate
- C Succinate to Fumarate
- D Fumarate to Malate
Correct answer: Succinate to Fumarate
The oxidation of the 4-carbon succinate into fumarate involves the removal of two hydrogens. These are accepted by FAD (Flavin Adenine Dinucleotide) to form FADH2.
Which intermediate of the Krebs cycle is a 5-carbon compound?
- A Citric acid (6C)
- B Alpha-ketoglutaric acid
- C Succinic acid (4C)
- D Oxaloacetic acid (4C)
Correct answer: Alpha-ketoglutaric acid
Alpha-ketoglutarate is the only 5-carbon intermediate in the cycle. It is formed after the first decarboxylation of isocitrate (6C).
For every two molecules of Pyruvic acid entering the mitochondria, how many NADH molecules are produced in the Krebs cycle specifically?
- A 3
- B 4
- C 6
- D 8
Correct answer: 6
One turn of the Krebs cycle (per pyruvate) produces 3 NADH. Since two pyruvates are formed from one glucose, the cycle turns twice, producing a total of 6 NADH.
The total number of FADH2 molecules produced from the complete aerobic oxidation of one glucose molecule is:
- A 1
- B 2
- C 4
- D 6
Correct answer: 2
One FADH2 is produced per turn of the Krebs cycle at the succinate to fumarate step. With two turns per glucose molecule, a total of 2 FADH2 molecules are generated.
What is the net yield of ATP/GTP through substrate-level phosphorylation for every Acetyl CoA that enters the TCA cycle?
- A 1
- B 2
- C 12
- D 38
Correct answer: 1
Each turn of the TCA cycle (driven by one Acetyl CoA) produces exactly one molecule of ATP (or GTP) directly through substrate-level phosphorylation during the succinyl-CoA to succinate conversion.
Which of the following is NOT a product of the Krebs cycle?
- A Carbon dioxide (CO2)
- B NADH + H+
- C FADH2
- D Lactic acid
Correct answer: Lactic acid
Lactic acid is a product of anaerobic fermentation in the cytoplasm. The Krebs cycle occurs under aerobic conditions in the mitochondria and produces CO2 and reduced coenzymes.
Which chemical reaction describes the 'regeneration' step of the Krebs cycle?
- A Citrate to Isocitrate
- B Fumarate to Malate
- C Malate to OAA
- D Succinate to Fumarate
Correct answer: Malate to OAA
The cycle is completed when malate is oxidized back to oxaloacetic acid (OAA). This allows OAA to pick up another Acetyl CoA molecule and start the cycle again.
The oxygen required for the 'aerobic' nature of the Krebs cycle is used directly in which step?
- A Isocitrate decarboxylation
- B Phosphorylation of GDP
- C Oxidation of Malate
- D None of these steps
Correct answer: None of these steps
Oxygen is not used directly in any step of the Krebs cycle. It is the final electron acceptor in the Electron Transport Chain; without it, the ETC stalls, leading to a shortage of NAD+ and FAD, which stops the Krebs cycle.
During the conversion of Alpha-ketoglutarate to Succinyl-CoA, which two processes occur simultaneously?
- A Hydration and Oxidation
- B Oxidation and Decarboxylation
- C Reduction and Carboxylation
- D Isomerization and Phosphorylation
Correct answer: Oxidation and Decarboxylation
This step is an oxidative decarboxylation. A molecule of CO2 is removed (decarboxylation) and NAD+ is reduced to NADH (oxidation of the substrate).
Which intermediate of the TCA cycle is used as a precursor for the synthesis of chlorophyll and cytochromes?
- A Citric acid
- B Alpha-ketoglutaric acid
- C Succinyl-CoA
- D Oxaloacetic acid
Correct answer: Succinyl-CoA
Succinyl-CoA is a vital metabolic intermediate used for the biosynthesis of porphyrins, which are key components of heme in cytochromes and the chlorophyll molecule.
If the Krebs cycle is inhibited at the malate dehydrogenase step, which substance will accumulate?
- A Oxaloacetic acid
- B Malic acid
- C Fumarate
- D Succinate
Correct answer: Malic acid
Malate dehydrogenase converts malic acid to oxaloacetic acid. If this enzyme is inhibited, malic acid cannot be converted and will increase in concentration.
The total number of CO2 molecules released from one molecule of glucose during the link reaction and the Krebs cycle combined is:
- A 2
- B 4
- C 6
- D 12
Correct answer: 6
One glucose yields two pyruvates. The link reaction releases 2 CO2 (1 per pyruvate) and the Krebs cycle releases 4 CO2 (2 per pyruvate), totaling 6 CO2 molecules, matching the 6 carbons in glucose.
The Krebs cycle is also referred to as the 'Amphibolic Pathway' because it is involved in:
- A Only catabolism (breakdown)
- B Only anabolism (synthesis)
- C Both catabolism and anabolism
- D Both aerobic and anaerobic respiration
Correct answer: Both catabolism and anabolism
It is catabolic because it breaks down acetyl groups to CO2, and anabolic because many of its intermediates (like alpha-ketoglutarate or OAA) are withdrawn for the synthesis of amino acids and other biomolecules.