A mathematical model for the growth of a single cell of E. coli on medium containing amino acid is presented. A mixture of purified amino acids (glutamate, aspartate, serine, tyrosine, and leucine) combined in the ratios found in a natural digest (casein) were employed as the nitrogen source. Each of these amino acids is the representative of a different family of amino acids. The transport mechanisms and assimilation routes for each amino acid were inserted into the prototype model. The enzyme activities and saturation constants used in the model were based on literature data. The maximum velocities for uptake systems were calculated from experimental data. The formation and homeostasis of amino acid pools were regulated through cross-control of the activities of biosynthetic enzymes and of membrane transport of exogenous nutrients. The size of each amino acid pool was determined with mass balance equations that included terms for a transport system, a biosynthesis system, a transaminase enzyme system for interchange between the amino acid families, and a consumption system. The predictions of the extended model with regard to nutrient concentrations and growth rates compared well with the experimental data.