{ diff(x1, t) = (1*k1*k4*x3/(k2 + x3)*x4/(k3 + x4) + 1*k1*k5*x5 + 1*k1*k6 + (-1)*k1*k7*x1)/k1,
  diff(x2, t) = ((-1)*k1*k11*x2 + 1*k1*k18*x6/(k19 + x6)*x5/(k20 + x5)*x3/(k21 + x3))/k1,
  diff(x4, t) = ((-1)*k1*k12*x4 + 1*k1*k14*x2)/k1,
  diff(x5, t) = ((-1)*k1*k10*x5 + 1*k1*k15*x7)/k1,
  diff(x6, t) = ((-1)*k1*k8*x6 + 1*k1*k13*x1)/k1,
  diff(x7, t) = ((-1)*k1*k9*x7 + 1*k1*k16*x6*k17/(k17 + x3))/k1 }