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