{ diff(x1, t) = (-1)*k1*(k2*x1*x3 - k3*x2)/k1,
  diff(x2, t) = (1*k1*(k2*x1*x3 - k3*x2) + (-1)*k1*(k4*x2*x4 - k5*x5))/k1,
  diff(x3, t) = (-1)*k1*(k2*x1*x3 - k3*x2)/k1,
  diff(x4, t) = (-1)*k1*(k4*x2*x4 - k5*x5)/k1,
  diff(x5, t) = 1*k1*(k4*x2*x4 - k5*x5)/k1 }