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 released.What is the best explanation for this observation?

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Quizplus · Accepted Answer

The observation suggests that the acetyl groups are being incorporated into a cyclical metabolic pathway, such as the citric acid cycle. At first, the produced CO₂ is not radioactive because it is not directly derived from acetyl-CoA. Instead, acetyl-CoA is first incorporated into citrate and oxaloacetate, which are then further metabolized into CO₂. Over time, as the citric acid cycle proceeds and radioactive carbon atoms are shuffled between different intermediates, eventually radioactive CO₂ is produced.

When Acetyl-CoA Containing Radioactively Labeled Carbon Atoms Is Fed to Cells,the