  Here, steady-state reaction networks are inspected from the viewpoint of
individual tagged molecules jumping among their chemical states upon the
occurrence of reactive events. Such an agent-based viewpoint is useful for
selectively characterizing the behavior of functional molecules, especially in
the presence of bimolecular processes. We present the tools for simulating the
jump dynamics both in the macroscopic limit and in the small-volume sample
where the numbers of reactive molecules are of the order of few units with an
inherently stochastic kinetics. The focus is on how an ideal spatial
"compartmentalization" may affect the dynamical features of the tagged
molecule. Our general approach is applied to a synthetic light-driven
supramolecular pump composed of ring-like and axle-like molecules that
dynamically assemble and disassemble, originating an average ring-through-axle
directed motion under constant irradiation. In such an example, the dynamical
feature of interest is the completion time of direct/inverse cycles of tagged
rings and axles. We find a surprisingly strong robustness of the average cycle
times with respect to the system's size. This is explained in the presence of
rate-determining unimolecular processes, which may, therefore, play a crucial
role in stabilizing the behavior of small chemical systems against strong
fluctuations in the number of molecules.
