{ diff(x1, t) = ((-1)*k15*(k1*x1 + k2*x5*x1/(k3 + x1)) + 1*k15*k11 + (-1)*k15*k13*x1)/k15,
  diff(x2, t) = (1*k15*(k1*x1 + k2*x5*x1/(k3 + x1)) + (-1)*k15*(k4*x2*x7 - k5*x3) + 1*k15*k6*x5)/k15,
  diff(x3, t) = (1*k15*(k4*x2*x7 - k5*x3) + (-1)*k15*(k9*x3 - k10*x4))/k15,
  diff(x4, t) = (1*k15*(k9*x3 - k10*x4) + (-1)*k15*k7*x8*x4/(k8 + x4))/k15,
  diff(x5, t) = (1*k15*k7*x8*x4/(k8 + x4) + (-1)*k15*k6*x5)/k15,
  diff(x6, t) = 1*k15*k6*x5/k15,
  diff(x7, t) = ((-1)*k15*(k4*x2*x7 - k5*x3) + 1*k15*k12 + (-1)*k15*k14*x7)/k15,
  diff(x8, t) = 0/k15 }