{ diff(x10, t) = 0,
  diff(x11, t) = 0,
  diff(x12, t) = 0,
  diff(x1, t) = k1*x7*x2/(k8 + x2) - k2*x1/(k8 + x1),
  diff(x3, t) = (k5 + x3)*x4/(k9 + x4) - k4*x1*x3/(k9 + x3) - k7*x3*x5/(k9 + x3),
  diff(x5, t) = k6*x7*x6/(k9 + x6) - k3*x3*x5/(k9 + x5),
  diff(x8, t) = (-x8)*1/5,
  diff(x9, t) = (-x9)*1/2 }