Question

as shown in figure 9.10 in the textbook, citrate is the initial carboxylic acid in the citric acid pathway and oxaloacctate is the final carboxylic acid. to determine whether the citric acid pathway was cyclic, hans krebs treated cells with radioactive oxaloacetate. what would have been the result of krebss experiment if the pathway were linear instead of circular?

Explanation:

If the citric acid pathway were linear, radioactive oxaloacetate (the final carboxylic acid in a linear - cycle assumption) combining with acetyl CoA (which enters the cycle) would produce radioactive citric acid (the initial carboxylic acid in the cycle). In a circular pathway, the radioactivity would be "spread out" more as the cycle progresses, but in a linear pathway, the immediate product (citric acid) from the combination of oxaloacetate and acetyl CoA would show radioactivity.

• Radioactive oxaloacetate does not produce radioactive acetyl CoA as acetyl CoA is mainly from pyruvate decarboxylation (not directly from oxaloacetate in a simple linear - cycle - related reaction considered here).
• Radioactive oxaloacetate + acetyl CoA would produce radioactive citric acid cycle intermediates (citric acid is the first intermediate), so the option saying no radioactive citric acid cycle intermediates is wrong.
• Glucose is not directly produced from the combination of oxaloacetate and acetyl CoA in the citric acid cycle (the citric acid cycle is for energy production from acetyl CoA oxidation, not for glucose synthesis from these two molecules in this context).

Answer:

Radioactive oxaloacetate + acetyl CoA would produce radioactive citric acid.