Fig. 10.

Illustration of the flow of label in the same example as Fig. 9, but with explicit flux directions. In contrast to the scenario in Fig. 9, labeling experiments mandate the modeling of diffusion fluxes in both directions. Specifically, the simpler model in Fig. 9 does not allow flow of label from X₄ to X₂ through D when labeled metabolite is fed to the pathway through X₃, but the figure here demonstrates that such flow is clearly possible