Serine and glycine metabolism is generated de novo from glycolysis through the oxidation of intermediate 3-phosphoglycerate. Serine and glycine are also transported into cells. Sarcosine and possibly threonine can also enter cells and be converted to glycine. The question mark denotes that threonine catabolism has been found to be utizilized in mammals including mice. Its analog in humans has not been identified. Dashed arrows denote multiple biochemical steps. Bi-directional arrows denote reversible steps. Abbreviations: PHGDH – phosphoglycerate dehydrogenase, GLDC – Glycine Decarboxylase, SHMT – Serine hydroxymethyltransferase, GNMT – Glycine N-methyltransferase, THF – Tetrahydrofolate, Me-THF – 5,10 Methylenetetrahydrofolate.