The concept of functional compartment of a bacterial cell is discussed throughout. On the basis of previous experimental results, three such compartments are recognized in E. coli: 1) a compartment, CA, in which the nutrient is consumed and the energy and carbon requirements of the cell are satisfied.; 2) a compartment, CM, in which the bulk of the biological materials is synthesized; 3) a compartment, CD, in which the chromosome is replicated and the cell division prepared. The performance of the compartment CA is examined and three basic points are established: 1) All along the exponential growth of the O.D., the glucose transport system is saturated and the cell mass increase rate is proportional to the mass itself. When saturation fails (at $1 \div 2$ mg/l residual glucose concentration), the nutrient inflow decreases so rapidly as to bring to a sudden stop CM. In this way, a point transition is pratically effected. 2) In a stationary saturated culture, each cell must have a residual internal pool of ATP, in absence of which the cell could not start metabolizing glucose again in a refreshing medium. On the other side, the glucose transport system cannot work when phosphoenolpyruvate is absent. In the refreshing medium, the enzyme hexokinase acts as a starter: it phosphorilates glucose, which enters the cell by diffusion, till the phosphoenolpyruvate concentration is increased and the transport system begins to work. 3) It seems very likely that the enzyme phosphofructokinase switches over the functional compartment CM. When CM is off, the phophogluconate pathway remains open, with reduced energy consumption: the compartment CD keeps on working, until the glucose from the medium and the internal pool of precursors are exhausted. In a refreshing medium, the phosphofructokinase switch is initially off, the phosphoenolpyruvate is produced via phosphogluconate and CD cannot begin to work, until a sufficient precursor pool is available.