  To study the fluctuations and dynamics in chemical reaction processes,
stochastic differential equations based on the rate equation involving chemical
concentrations are often adopted. When the number of molecules is very small,
however, the discreteness in the number of molecules cannot be neglected since
the number of molecules must be an integer. This discreteness can be important
in biochemical reactions, where the total number of molecules is not
significantly larger than the number of chemical species. To elucidate the
effects of such discreteness, we study autocatalytic reaction systems
comprising several chemical species through stochastic particle simulations.
The generation of novel states is observed; it is caused by the extinction of
some molecular species due to the discreteness in their number. We demonstrate
that the reaction dynamics are switched by a single molecule, which leads to
the reconstruction of the acting network structure. We also show the strong
dependence of the chemical concentrations on the system size, which is caused
by transitions to discreteness-induced novel states.
