The production of biomass or a desired product from a cell can be improved by inducing conversion of ATP to ADP without primary effects on other cellular metabolites or functions which is achieved by expressing an uncoupled ATPase activity in said cell and incubating the cell with a suitable substrate to produce said biomass or product. This is conveniently done by expressing in said cell the soluble part (F¿1?) of the membrane bound (F¿0?F¿1? type) H?+¿-ATPase or a portion of F¿1? exhibiting ATPase activity. The organism from which the F¿1? ATPase or portions thereof is derived, or in which the F¿1? ATPase or portions thereof is expressed, may be selected from prokaryotes and eukaryotes. In particular the DNA encoding F¿1? or a portion thereof may be derived from bacteria and eukaryotic microorganisms such as yeasts, other fungi and cell lines of higher organisms and be selected from the group consisting of the gene encoding the F¿1? subunit $g(b) or a portion thereof and various combinations of said gene or portion with the genes encoding the other F¿1? subunits or portions thereof. The method can be used i.a. for optimizing the formation of biomass or a desired product by a cell by expressing different levels of uncoupled ATPase activity in the cell, incubating the cell on a suitable substrate, measuring the conversion rate of substrate into biomass or the desired product at each level of ATPase expression, and choosing a level of ATPase expression at which the conversion rate is optimized.