The Tricarboxylic Acid Cycle Quiz
The tricarboxylic acid cycle — also called the Krebs’ cycle — is the central engine of aerobic respiration. Inside the mitochondrial matrix, the acetyl group of acetyl CoA is dismantled step by step, releasing carbon dioxide and conserving energy as reduced coenzymes.
In this quiz you will test your grasp of:
- The condensation of the acetyl group with oxaloacetic acid to form citric acid
- Isomerisation of citrate to isocitrate and the two successive decarboxylations
- GTP synthesis by substrate level phosphorylation
- Formation of NADH + H+ and FADH2 at specific points in the cycle
- Replenishment of oxaloacetic acid and regeneration of NAD+ and FAD+
Work carefully through each question — the numerical accounting of the cycle is where most learners slip.
Q1. The TCA cycle begins with the condensation of the acetyl group with which compound?
The TCA cycle starts with the condensation of the acetyl group with oxaloacetic acid (OAA) and water to yield citric acid.
Q2. Which enzyme catalyses the condensation of the acetyl group with oxaloacetic acid?
The reaction forming citric acid from acetyl group and OAA is catalysed by the enzyme citrate synthase, and a molecule of CoA is released.
Q3. When the acetyl group condenses with oxaloacetic acid and water, which molecule is released?
During the citrate synthase reaction, a molecule of CoA is released as citric acid is formed.
Q4. Citrate formed in the first step of the cycle is immediately converted into which intermediate?
Citrate is isomerised to isocitrate before the decarboxylation steps begin.
Q5. After the isomerisation of citrate, the cycle proceeds through two successive steps of which type of reaction?
Following isomerisation of citrate to isocitrate, two successive steps of decarboxylation lead to the formation of α-ketoglutaric acid and then succinyl-CoA.
Q6. The two successive decarboxylations in the cycle lead to the formation of α-ketoglutaric acid and then which compound?
The two decarboxylation steps produce α-ketoglutaric acid and then succinyl-CoA.
Q7. During the conversion of succinyl-CoA to succinic acid, which molecule is synthesised?
During the conversion of succinyl-CoA to succinic acid a molecule of GTP is synthesised by substrate level phosphorylation.
Q8. The synthesis of GTP during the conversion of succinyl-CoA to succinic acid is an example of which process?
The formation of GTP in this step is described as substrate level phosphorylation.
Q9. In the coupled reaction following GTP synthesis, GTP is converted to GDP with the simultaneous synthesis of which molecule?
In a coupled reaction GTP is converted to GDP with the simultaneous synthesis of ATP from ADP.
Q10. How many points in the TCA cycle reduce NAD+ to NADH + H+?
There are three points in the cycle where NAD+ is reduced to NADH + H+.
Q11. At how many points in the cycle is FAD+ reduced to FADH2?
There is one point in the cycle where FAD+ is reduced to FADH2.
Q12. Which compound is described as the first member of the cycle that must be continually replenished?
The continued oxidation of acetyl CoA via the TCA cycle requires the continued replenishment of oxaloacetic acid, the first member of the cycle.
Q13. Besides replenishment of oxaloacetic acid, what else is required for continued oxidation of acetyl CoA via the TCA cycle?
The cycle also requires regeneration of NAD+ and FAD+ from NADH and FADH2 respectively.
Q14. In which part of the mitochondrion does the TCA cycle take place?
The TCA cycle operates in the matrix of the mitochondrion, while the electron transport system is on the inner membrane.
Q15. For one glucose molecule, how many molecules of ##NADH + H+ and## FADH2 are synthesised in the TCA cycle, besides two ATP?
The lesson states that glucose has been broken down to release ##\ce{CO2}## and eight molecules of ##NADH + H+ and## two of FADH2 besides just two molecules of ATP in the TCA cycle.