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