Under appropriate conditions and given sufficient time, a single biochemical reaction carried out in a test tube eventually reaches equilibrium, at which the concentrations of reactants and products do not change with time because the rates of the forward and reverse reactions are equal. Within cells, however, many reactions are linked in pathways in which a product of one reaction is not simply reconverted via a reverse reaction to the reactants. For example, the product of one reaction might serve as a reactant in another, or it might be pumped out of the cell. In this more complex situation, the original reaction can never reach equilibrium because some of the products do not have a chance to be converted back to reactants. Nevertheless, in such non-equilibrium conditions, the rate of formation of a substance can be equal to the rate of its consumption, and as a consequence, the concentration of the substance remains constant over time. In such circumstances, the system of linked reactions for producing and consuming that substance is said to be in a steady state (Figure 2-23). One consequence of such linked reactions is that they prevent the accumulation of excess intermediates, protecting cells from the harmful effects of intermediates that are toxic at high concentrations. When the concentration of a product of an ongoing reaction is not changing over time, it might be a consequence of a state of equilibrium, or it might be a consequence of a steady state. In biological systems, when metabolite concentrations, such as blood glucose levels, are not changing with time—a condition called homeostasis—it is a consequence of a steady state rather than equilibrium.
FIGURE 2-23 Comparison of reactions at equilibrium and at steady state. (a) In the test tube, a biochemical reaction (A → B) eventually reaches equilibrium, at which the rates of the forward and reverse reactions are equal (as indicated by the reaction arrows of equal length). (b) In metabolic pathways within cells, the product B is commonly consumed—in this example, by conversion to C. A pathway of linked reactions is at steady state when the rate of formation of the intermediates (e.g., B) equals their rate of consumption. As indicated by the unequal length of the arrows, the individual reversible reactions constituting a metabolic pathway do not reach equilibrium. Moreover, the concentrations of the intermediates at steady state can differ from what they would be at equilibrium.