In this contribution we present a population balance modeling approach for the production of the biopolymer poly(β-hydroxybutyrate) in Ralstonia eutropha. The population balance model is based on a dynamic single cell model, which accounts for cell internal regulation by means of the cybernetic modeling approach. The change of internal coordinates is controlled by cybernetic control variables. Depending on available substrates and internal composition the population balance model is therefore able to switch between growth, synthesis of biopolymer and metabolization of biopolymer. The latter one was neglected in an earlier contribution, but is crucial for overall dynamic behavior.In a first step we present a extended two-dimensional population balance model which includes metabolization of biopolymer and considers the cell internal biopolymer and residual biomass as internal coordinates. The two-dimensional population balance model includes cell internal regulation by means of cybernetic control variables.Since concentration of internal biopolymer and amount of residual biomass are costly to determine, we discuss in a second step a reduction of the two-dimensional to a one-dimensional population model by means of correlating cell size with biopolymer concentration.