{ diff(x1, t) = (1*k6*k1*(1 - x1)/(k4 + x3) + (-1)*k6*k2*x3*x1/(k5 + x1))/k6,
  diff(x2, t) = (1*k6*x1*x1 + (-1)*k6*k7*x2)/k6,
  diff(x3, t) = (1*k6*x2 + (-1)*k6*k3*x3*(1 - x1)/(k4 + x3))/k6 }