Figure 7: The s₂–c₂ phase-plane trajectory (with nullclines) for the auxiliary reaction mechanism (47)–(48).

The thick black curve is the numerically-integrated solutions to the mass action equations (49a)–(49d). The broken red curve is the c₂-nullcline, and the broken blue curve is the fixed s₂-nullcline ($N_{s_2}^{\max }$, given by (56)). The phase–plane trajectory initially moves towards $N_{s_2}^{\max }$, then moves up $N_{s_2}^{\max }$ before moving back down the c₂-nullcline. The constants (without units) used in the numerical simulation are: $e_1^0=1$, $s_1^0=1000$, k₁ = 1, k₂ = 1, k₃ = 1, k₋₃ = 1, k₄ = 2, $e_2^0=100$ and k₋₁ = 1. Curves were scaled by their numerically obtained maximum values.