Solution-Find out the major pathway of glycine degradation

In order to determine the major pathway of glycine degradation, you devise an experiment in which mice are fed 13C(alpha)-labeled glycine, and the livers are then isolated and radioactive intermediates detected by GC-MS. Interestingly, you find that the amino acid serine contains the most radioactivity, and that both the Calpha and Cbeta carbons of the serine residue are labeled with 13C. Explain these observations.