In order to find out the main 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 the GC-MS. Interestingly, you find out that the amino acid serine includes the most radioactivity, and that both the Calpha and Cbeta carbons of the serine residue are labeled with 13C. Describe these observations.