When acetyl-CoA containing radioactively labeled carbon atoms is fed to cells, the CO2 produced is not initially radioactive. However, molecules of citrate and oxaloacetate are radioactive. Only after some time has passed will radioactive CO2 be released. What is the best explanation for this observation?
A. Acetyl groups are not metabolized directly into CO2, but enter a cyclical metabolic pathway.
B. Acetyl groups are not metabolized directly into CO2, but enter a linear metabolic pathway.
C. Acetyl groups first combine with citrate and oxaloacetate before being metabolized into CO2.
D. Acetyl groups are metabolized directly into CO2.
Answer: A
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