The hybrid cybernetic approach (Kim et al., 2008; Song et al., 2009; Song & Ramkrishna, 2009) is used for modeling of recombinant yeast consuming glucose and xylose. The hybrid cybernetic model (HCM) incorporates the concept of elementary modes (EMs) (Schuster et al., 2000) into the cybernetic framework. EM is a metabolic pathway (or subnetwork) composed of a minimal set of reactions supporting a steady state operation of metabolism. Any feasible metabolic state can be represented by nonnegative combinations of EMs. HCM views EMs as cell’s metabolic options, the choice of which is optimally modulated under dynamic environmental conditions such that a prescribed metabolic objective (such as the total carbon uptake flux) is maximized.