{ diff(x1, t) = ((-1)*k7*k4*k5*x1*x3/x5 + 1*k7*k2*x4)/k7,
  diff(x2, t) = (1*k7*k4*k5*x1*x3/x5 + (-1)*k7*k3*x2)/k7,
  diff(x3, t) = (1*k7*k3*x2 + (-1)*k7*k4*x3)/k7,
  diff(x4, t) = (1*k7*k4*x3 + (-1)*k7*k2*x4)/k7,
  diff(x5, t) = 0/k7 }