{ diff(x1, t) = (1*k16*k10 + (-1)*k16*(k11*x3*x1/(k6 + x1) + k7*x1))/k16,
  diff(x2, t) = (1*k16*x1/(k5 + x1)*k12*(1 - x2)/(k1 + 1 - x2) + (-1)*k16*k8*x2/(k1 + x2))/k16,
  diff(x3, t) = (1*k16*x2*k13*(1 - x3)/(k3 + 1 - x3) + (-1)*k16*k9*x3/(k4 + x3))/k16 }