{ diff(x1, t) = (-1)*k11*k2*x1/k11,
  diff(x2, t) = (1*k11*k2*x1 + (-1)*k11*k2*x2)/k11,
  diff(x3, t) = (1*k11*k2*x2 + (-1)*k11*k10*x3)/k11,
  diff(x4, t) = (1*k11*k8 + (-1)*k11*k5*k7^k8/(k7^k8 + x5^k8)*x4*x3)/k11,
  diff(x5, t) = (1*k11*k6*x4 + (-1)*k11*k6*x5)/k11 }