Final answer:
CO₂ is produced as a waste product in the conversion of pyruvate to acetyl-CoA and during the Krebs cycle. The electron transport chain does not produce CO₂, and while fermentation does produce CO₂, it is not part of cellular respiration. The compound produced only in the Krebs cycle is FADH2.
Step-by-step explanation:
Stages of Cellular Respiration Producing CO₂
The stages of cellular respiration that produce CO₂ as a waste product are the Krebs cycle and the conversion of pyruvate to acetyl-CoA, which occurs before the Krebs cycle. During glycolysis, a molecule of glucose is split into two three-carbon molecules of pyruvate. This process does not release CO₂. However, when pyruvate is oxidized to form acetyl-CoA, one carbon from each pyruvate molecule is released as CO₂. This transformation is often considered part of the preparatory phase leading into the Krebs cycle. It is within the Krebs cycle where CO₂ is produced at multiple steps as the acetyl groups are fully oxidized. In contrast, the electron transport chain does not produce CO₂; it utilizes the electrons from NADH and FADH2 to generate ATP. Fermentation also produces CO₂, but this process occurs only in the absence of oxygen and is not part of cellular respiration.
To address the accompanying question regarding which compound is produced only in the Krebs cycle (referencing the options given), the answer is FADH2. While ATP and NADH are also products of the Krebs cycle, they are produced during glycolysis as well. However, FADH2 is generated only during the Krebs cycle.