Final answer:
a) By showing immediate appearance. The citrate cycle was shown to be cyclic, not linear, by using carbon-14 labeling to demonstrate the immediate appearance of radioactivity in carbon dioxide,
Step-by-step explanation:
The carbon-14 labeling experiments demonstrated that the citrate cycle is a cycle and not a linear pathway by tracking the intermediary product levels and showing the immediate appearance of radioactivity in the carbon dioxide product.
When carbon-14 labeled compounds were introduced into the citric acid cycle, the radioactivity was expected to appear immediately in carbon dioxide if the process was cyclic.
Confirmation of the cycle was evident because carbon-14 was incorporated into citrate and, as the cycle progressed, the carbon from citrate was converted into carbon dioxide and then released, thereby appearing in carbon dioxide.
This demonstrated that the citrate was not irreversibly transformed into carbon dioxide but rather went through a series of reactions that reused and regenerated intermediaries, confirming the cyclical nature of the citric acid cycle.
In carbon-14 labeling experiments, scientists can track the appearance of radioactivity in the carbon dioxide product to determine if the citrate cycle is a cycle or a linear pathway.
They do this by analyzing the compounds present in the pathway and the amount of each compound at different time intervals. If the citrate cycle is a cycle, the radioactivity will appear in intermediary products, such as citrate or isocitrate, before appearing in the carbon dioxide product.
This demonstrates the cyclical nature of the pathway.