Organic compounds containing radioactive atoms can be used to follow the biosynthesis of molecules in cells and organisms. In these experiments, the amount of radioisotope in proteins derived from the cells or organisms is monitored. The administration of 14C-labeled 4-hydroxyproline to mice results in no appearance of 14C in the animals' collagen fibers. However, the administration of 14C-labeled proline does result in the appearance of 14C in the collagen fibers of mice. Explain these results.