Fig. 4. A typical carbon labelling experiment (CLE) workflow. (A) A metabolic model of cellular metabolism is predicted. (B) Cell cultivation and ¹³C-CLE are conducted, sampling is performed to have information about exchange rates between cells and environment; when metabolic steady state and isotopic steady are reached, quenching and extraction are carried out. (C) MS or NMR spectroscopy-based isotopic analysis are performed for labelling measurements. (D) Cell fluxes are assessed by integration of internal and external rate calculations.