Fig. The validity of the QSSA for slow indicator reactions. The phase-space trajectory is initially swept away from the c 2 -nullcline (broken red curve) in panel (a). The solid black curve is the numerical solution to the mass action equations (7a)-(7d). In panel (b) the QSSA is essentially valid in neighborhoods containing the origin. Note that c 2 and s 2 as well as the c 2 -nullcline have been scaled by their maximum values for illustrative purposes.