Experimental observations of cell size variations in the proliferating rhodophyte Porphyridium cruentum cultured under fully controlled conditions showed significant decreases from inoculation to a steady state in the chemostat with 0.23 d(-1) dilution rate and to a minimum in batch, dropping in size by ratios of over 10. To numerically simulate these variations, we assumed that the cell is made up of two categories of components that behave differently during the interphase and mitosis. These have been called essential (EC) and accessory (AC) components. It is assumed that the cell divides once the EC have doubled in size, regardless of the AC's state. The experimental cell weight time courses were correctly simulated by a model of synchronous cell kinetics based on these assumptions. The EC's specific growth rate was 1.5 times that of the whole cell, when no limitation occurred. The increase in cell weight observed during batch cultures after nutrient exhaustion was suitably simulated by assuming that EC growth stops when a limiting nutrient is exhausted. Several parameters characterizing the cell kinetics were defined, particularly the minimum minimorum EC or cell weight (26 and 15 pg for chemostat and batch, respectively), which was influenced by the cultivation method, and the maximum whole cell weight (224 to 244 pg), which depended on the inoculum's age. The influence of culture conditions on the amount of essential and accessory components contained in a cell was examined. A new approach was developed with respect to these compartments to determine the most suitable strategy and conduct a predictive approach for valuable molecule production.