[Campbell Biology P.222] According to the description in Figure 9.11, what is a key characte... | Practice Question

According to the description in Figure 9.11, what is a key characteristic regarding the fate of the carbon atoms that enter the citric acid cycle from acetyl CoA?

  • A: They exit the cycle as CO2 in the same turn they enter.
  • B: They remain in the cycle, occupying a different location in the molecules on their next turn, before exiting as CO2.
  • C: They are completely removed from the cycle as CO2 by step 5 due to succinate's symmetry.
  • D: They are immediately incorporated into oxaloacetate and regenerate it in the same turn.

Explanation

The text states: 'Notice that the carbon atoms that enter the cycle from acetyl CoA do not leave the cycle in the same turn. They remain in the cycle, occupying a different location in the molecules on their next turn, after another acetyl group is added. Therefore, the oxaloacetate regenerated at step 8 is made up of different carbon atoms each time around.'