{ diff(x1, t) = (1*k11*k2 + (-1)*k11*k1*x1 + (-1)*k11*k3*x1*x2)/k11,
  diff(x2, t) = (1*k11*k7*x3 + (-1)*k11*k8*x2)/k11,
  diff(x3, t) = (1*k11*k3*x1*x2 + (-1)*k11*k4*x3 + (-1)*k11*k5*x4*x3)/k11,
  diff(x4, t) = (1*k11*k9*x5 + (-1)*k11*k10*x4)/k11,
  diff(x5, t) = (1*k11*k5*x4*x3 + (-1)*k11*k6*x5)/k11 }