Expanded mathematical model with the additional contribution of Golgi PtdIns4P. (A–I) Calculations from an expanded model to include the gray arrows as well as the black arrows of Fig. 1. Several time-varying changes were assumed and tested for the rate constants for PI4K of the PM (A) and of the Golgi (B) during Oxo-M (shaded area). Corresponding simulated time courses of PM PtdIns (C), Golgi PtdIns (D), PM PtdIns4P (E), Golgi PtdIns4P (F), PM PtdIns(4,5)P₂ (G), Ins(1,4,5)P₃ (H), and PM DAG (I). The concentration of PtdIns at the ER is held constant. _G indicates chemical species in the Golgi compartment.