Schematic diagrams of models 1–4. Compartments are indicated by dashed lines, reactions by single-headed curved arrows, and exchange between compartments by double-headed straight arrows. All models have compartments representing the solution, cytosol, and mitochondrial intermembane space (IMS). Models 3 and 4 include a fourth compartment (C4). The reactions considered are mitochondrial ATP synthesis in the mitochondrial matrix leading to the conversion of ADP to ATP in the mitochondrial IMS (ATPsyn), ATP consumption by ATPases (ATPase1 and ATPase2), and ATP synthesis by endogenous pyruvate kinase (PK; PKend1 and PKend2) and exogenous PK. The following exchanges between compartments were calculated: solution and cytosol (sol-cyt), IMS and cytosol through the outer mitochondrial membrane (MoM), cytosol and C4 (cyt-C4), and IMS and C4 (IMS-C4). PEP, phosphoenol pyruvate.