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