{ diff(x1, t) = k3/(k11 + x2) - k8*x1,
  diff(x2, t) = k1*((k4 + k5*x1)*x2*x3/(k12 + x1^2) - k9*x2),
  diff(x3, t) = k1*k2*(k6*x1 - (k10*x3 + k7*x1*x3/(k13 + x1))) }