Kaplan Nursing Entrance Practice Exam

What type of molecule is added to a 4-carbon molecule at the beginning of the Krebs cycle?

1. 1-carbon molecule

2. 2-carbon molecule

3. 3-carbon molecule

4. 5-carbon molecule

The correct answer is: 2-carbon molecule

At the beginning of the Krebs cycle, a 4-carbon molecule, which is oxaloacetate, combines with a 2-carbon molecule called acetyl-CoA. This reaction forms a 6-carbon molecule known as citrate, initiating the cycle of reactions that will ultimately lead to energy production in the form of ATP along with the release of CO2 and high-energy electron carriers, NADH and FADH2. The 2-carbon unit provided by acetyl-CoA is critical because it introduces the necessary carbon atoms to continue the cycle of transformations that characterize the Krebs cycle. Each turn of the cycle begins with the interaction of this 2-carbon molecule and the existing 4-carbon compound. This specific coupling ensures a continuous flow of reactions that feed into cellular respiration processes. The other options do not apply because the Krebs cycle specifically begins with a 2-carbon addition to the existing 4-carbon molecule. A 1-carbon molecule is too small to participate in this particular cycle, while 3-carbon and 5-carbon molecules do not play a role at the starting point of the Krebs cycle in the same manner as the 2-carbon acetyl-CoA.