  Text-book concepts of diffusion- versus kinetic-control are well-defined for
reaction-kinetics involving macroscopic concentrations of diffusive reactants
that are adequately described by rate-constants -- the inverse of the
mean-first-passage-time to the reaction-event. In contradistinction, an open
important question is whether the mean-first-passage-time alone is a sufficient
measure for biochemical reactions that involve nanomolar reactant
concentrations. Here, using a simple yet generic, exactly-solvable model we
study the conspiratory effect of diffusion and chemical reaction-limitations on
the full reaction-time distribution. We show that it has a complex structure
with four distinct regimes delimited by three characteristic time scales
spanning a window of several decades. Consequently, the reaction-times are
defocused: no unique time-scale characterises the reaction-process, diffusion-
and kinetic-control can no longer be disentangled, and it is imperative to know
the full reaction-time distribution. We introduce the concepts of geometry- and
reaction-control, and also quantify each regime by calculating the
corresponding reaction depth.
