Aerobic chemostat cultures of S. cerevisiae were performed at different diln. rates under energy (glucose) limitation or at various extents of energy excess imposed by a N limitation. Energy excess induced metabolic uncoupling under steady-state conditions. The sp. EtOH prodn. rate was always higher the lower the feed medium N concn. throughout the whole range of diln. rates tested. In addn., the respiratory rate also increased under N limitation, for at least as long as the sp. O2 consumption rate was below the max. capacity. These results imply that the ATP yield (YATP in g biomass per mol ATP) and/or the amt. of ATP produced, i.e. the P/O ratio, must be able to change. By assuming a const. YATP of 16 g/mol, a decrease in the P/O ratio from 1 to 1.5 at low diln. rates and glucose limitation down to only 0.1-0.2 at high diln. rates and N limitation could be calcd. If, instead, a fixed P/O ratio of 1.0 was assumed, the YATP decreased from .apprx.20 down to <10 g/mol during these different conditions. The heat yield values, i.e., the amt. of heat produced per amt. of biomass formed, increased dramatically when the cells were subjected to a N limitation, indicating less efficient growth in terms of conserving substrate energy as biomass under energy excess compared to energy-limiting conditions. [on SciFinder (R)]