{ diff(x1, t) = (1*k1*k2 + 1*k1*k3*x1 + (-1)*k1*k4*x2*x1/(k5 + x1) + (-1)*k1*k6*x3*x1/(k7 + x1))/k1,
  diff(x2, t) = (1*k1*k8*x1 + (-1)*k1*k10*x2/(k9 + x2))/k1,
  diff(x3, t) = (1*k1*k11*x1 + (-1)*k1*k13*x3/(k12 + x3))/k1 }