Chapter_1 Introduction
The rate at which any given reaction proceeds depends on the specifics of that reaction, some of which are determined according to the particulars of its chemistry and other of which are subject to some control by the cell (by varying enzyme copy number, for example), and also on concentrations of that reaction's reactions and products. Given reaction rates, metabolite concentrations and reaction fluxes can be tracked; this process is called reaction kinetics. However, parameterizing all of these aspects of kinetics, especially within a cellular environment, is problematic. Hence, rather, a steady state assumption is commonly introduced whereby metabolite concentration and reaction fluxes are assumed constant in time. Under the steady state assumption, reaction fluxes must be balanced: the rate at which molecules of any given metabolite are created must equal the rate at which molecules of that same metabolite are depleted. Kinetics can then be replaced, in part, by this flux balance constraint. Admissible (feasible): a vector of fluxes, one component for each network reaction, is called admissible (feasible) if, for each metabolite in the network, the flux of newly created molecules equals the flux of newly reacted ons. the complete set of such admissible vectors represents the complete steady state metabolic potential of the metabolic network and of the cell which houses that network.