[Campbell Biology P.222] Step 5 of the citric acid cycle involves the displacement of CoA by... | Practice Question

Step 5 of the citric acid cycle involves the displacement of CoA by a phosphate group, forming GTP from GDP, which can then generate ATP. What is the primary significance of this step for cellular energy production?

  • A: It is the main pathway for direct ATP synthesis in the citric acid cycle through oxidative phosphorylation.
  • B: It represents a substrate-level phosphorylation event, directly yielding energy in the form of GTP (and indirectly ATP).
  • C: It is crucial for regenerating NAD+ and FAD, which are essential for continued cycle operation.
  • D: It is the only step where CO2 is released, ensuring carbon atom removal from the cycle.

Explanation

Step 5 is an example of substrate-level phosphorylation, where a phosphate group is directly transferred from a substrate molecule succinyl CoA to GDP to form GTP, which can then be used to generate ATP. This is a direct energy yield within the cycle, distinct from the larger ATP production of oxidative phosphorylation.