Fig. 4.

Flux diagram for a metabolite. The intracellular concentration of carbon ([C]_in) is determined by the external concentration [C]_out, the plasma membrane transport rates (ν₁ for influx; ν₆ for efflux), the rate of biosynthesis (ν₂) and catabolism (ν₄) in the cytosol, and the rates of import (ν₃) and release (ν₅) from/to one or several intracellular compartments. Regulatory effects on any of the parameters will affect steady-state concentrations. Fluorescence resonance energy transfer (FRET) sensors can determine both steady states as well as the rates of accumulation and elimination when external concentrations are changed or when any of the parameters is altered by genetic or chemical modification. X, precursor; Y, synthesized compound.