The function of the central carbon metabolism (metabolism of glucose) has been fine-tuned to exactly meet the needs of the building blocks and Gibbs free energy in conjunction with cell growth. There is therefore tight regulation of the fluxes through the central carbon metabolism.
The flux in a reaction can be defined based on one of three things
*The activity of the enzyme catalysing the reaction
*The properties of the enzyme
*The metabolite concentration affecting enzyme activity.
Considering the above, the metabolic fluxes can be described as the ultimate representation of the cellular phenotype when expressed under certain conditions.