Unlike the other assimilatory pathways, the serine cycle uses carboxylic acids and amino acids as intermediates instead of carbohydrates. First, 2 molecules of formaldehyde are added to 2 molecules of the amino acid glycine. This produces two molecules of the amino acid serine, the key intermediate of this pathway. These serine molecules eventually produce 2 molecules of 2-phosphoglycerate, with one <chem>C3</chem> molecule going towards biomass and the other being used to regenerate glycine. Notably, the regeneration of glycine requires a molecule of <chem>CO2</chem> as well, therefore the Serine pathway also differs from the other 3 pathways by its requirement of both formaldehyde and <chem>CO2</chem>.