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