Question

Researchers were analyzing cellular respiration in humans. They labeled the carbon atoms (C) in glucose with a tag that allowed them to follow where carbons went. They then made some cupcakes using the glucose that contained the labeled carbon atoms. They asked human test subjects to eat those cupcakes and attempted to follow where the labeled carbon atoms went. If the researchers were able to collect tissue samples from the human test subjects, where would they find those labeled carbons at the END of cellular respiration? Select only ONE answer choice.

a. They would find those labeled carbon atoms in newly produced G3P molecules
b. They would find those carbon atoms in CO₂ molecules
c. They would find those carbon atoms in molecules of pyruvate
d. They would find those carbon atoms in molecules of ATP
e. None of the above

Answer 1

The labeled carbon atoms from glucose consumed in the cupcakes would be found in CO2 molecules at the end of cellular respiration, as they are released during the Citric Acid Cycle (Krebs cycle) when glucose is fully oxidized. The correct answer is option b.

Step-by-step explanation:

When analyzing the cellular respiration of glucose (C6H12O6) in humans, the fate of the carbon atoms during this process can be traced. After ingesting glucose, during cellular respiration, the glucose molecule undergoes a series of transformations. The initial step of glycolysis breaks glucose into two molecules of pyruvate, each with three carbon atoms. As the process continues, each pyruvate is converted into acetyl CoA, which is then processed through the Citric Acid Cycle (also known as the Krebs cycle). In this cycle, the carbon atoms are eventually released as carbon dioxide (CO2).

The explicit answer to the question is found in our understanding that during the second stage of glucose metabolism, specifically the Citric Acid Cycle, each carbon atom is eventually released as part of a molecule of CO2. This occurs after acetyl CoA merges with oxaloacetate to form citrate, which then goes through a series of reactions that remove carbon atoms, releasing them as CO2.

Therefore, after complete oxidation, the labeled carbon atoms originally found in the glucose would be found in CO2 molecules at the END of cellular respiration.

Given these facts, the correct answer to the question of where researchers would find the labeled carbon atoms at the end of cellular respiration if they could collect tissue samples from the human test subjects is option b: They would find those carbon atoms in CO2 molecules.