[../../tail.htm]To maintain the life of a cell, biochemical reactions cannot reach equilibrium state; otherwise, the reactions will not procede further. Fortunately, the living cell is an open system. Nutrients can constantly flow in and the waste can constantly flow out. There are all kinds of regulatory pathways to keep the input of a reaction is about equal to the output of a reaction. As a result, the metabolites can be kept at a constant level. This constancy is not established by equilibrium; it is a steady state. Although this state is apparently similar to equilibrium, i.e. quasi-equilibrium, it is established by kinetics. When the reaction rates of the input or output reactions are altered by enzyme activity, the steady state level will change. This phenomenon is quite different from the equilibrium, in which the forward and reverse reaction rates will both be changed by the enzyme. In other words, enzyme will not change the equilibrium level.
In contrast, a closed system, such as the biochemical reaction in a test tube or any in vitro experiment, will reach equilibrium because there is no mass exchange with the environment, i.e. the surrounding of this test tube. The product will accumulate in a closed system and increase the reverse reaction rate. Thus, both the substrate and product concentration will reach a constant level, which is determined by the equilibrium constant of this reaction. Since the in vitro experiments are always done in the test tube, the reaction will finally reach equilibrium. If you are not aware of difference between in vitro and in vivo reactions, you may misinterprete your results.
Last updated on 02/29/04