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