The schemes of kinetic models used as a base for simulation of isotopologue distribution. Metabolites are connected by biochemical reactions represented by arrows. Various colors indicate metabolites and reactions of specific pathways: green, glycolysis/gluconeogenesis; red, pentose phosphate pathways; orange, TCA cycle. The metabolites enclosed in ellipses are considered to be in fast equilibrium. (A), the basic model that includes one pool of hexose phosphates common for glycolysis and gluconeogenesis. (B), the model that includes also the additional pool of hexose phosphates (blue) that represents channeling in gluconeogenesis. Abbreviations are explained in the list of abbreviations in the text.