Final answer:
Carbon-14 labeling experiments demonstrated the cyclical nature of the citrate cycle by showing the progressive appearance of radioactivity in carbon dioxide, which indicated the reuse of compounds within the cycle.
Step-by-step explanation:
The use of carbon-14 labeling in experiments provided crucial evidence that the citrate cycle, also known as the Krebs cycle or TCA cycle, is a cyclical process, rather than a linear pathway. This was showcased through the appearance of radioactivity in carbon dioxide molecules as byproducts of the cycle. In these studies, scientists exposed plants to carbon dioxide enriched with the radioactive isotope C-14, then measured the radioactivity within different organic compounds over time.
The citrate cycle involves the oxidation of pyruvate, derived from glucose via glycolysis, to carbon dioxide. The process begins with the formation of citrate from acetyl CoA and oxaloacetate and progresses through multiple steps where compounds are oxidized and decarboxylated. Each step is numbered and in studies highlighting them, carbon atoms from acetyl CoA are marked for identification.
Using carbon-14 as a tracer, scientists observed the distribution and amount of radioactivity in different cycle intermediates over time. The cyclical nature of the pathway was confirmed as the labeled carbon atoms from carbon-14 appeared in carbon dioxide following several rounds of the cycle, rather than immediately. This indicated the reuse of compounds within the cycle and proved that the citrate cycle is indeed a cycle that repeatedly utilizes starting materials rather than consuming them in a straight line.