In this research, the physiology of the bakers' yeast which is an important ingredient of the Turkish staple food, bread, was studied. This study was based on the determination of the macromolecular composition of bakers' yeast under different growth conditions and of its response to different stress applications. This study can be divided into two major groups; continuous and fed-batch fermentation experiments. In continuous fermentation studies, the cell's macromolecular composition was determined for different dilution rates. Also, the cellular response to heat shock in terms of trehalose and protein was determined at constant dilution rate. It was observed that, the amount of storage carbohydrates -trehalose and glycogen- decreased whereas the amount of RNA and protein increased with increasing dilution rates. The second group of experiments comprised of fed-batch experiments. For this purpose, the steady-state cells grown continuously at a dilution rate of 0.1 $h^{-1}$ were stressed and at the same time switched to fed-batch fermentation. In this set of experiments, the effects of nutrient starvation and heat shock were studied separately and also in a combined manner. The results obtained from nutrient starvation experiments, showed that the highest trehalose accumulation was achieved when the cells were starved for both nitrogen and phoshate. In the cases where nutrient starvation and heat shock were applied together, trehalose accumulation was higher. The highest amount of trehalose accumulation was achieved when nitrogen-starvation together with heat shock and also nitrogen-phosphate starvation together with heat shock was applied.