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Not all of the reactions that consume the carbon and energy source are directly coupled with growth. Motility requires an expenditure of adenosine triphosphate ( ATP) ( Sherris et nl., 1957). The accumulation of solutes to a higher concentration in the cell than in the medium requires an initial expenditure of energy (Kepes, 1960; Patlak, 1961 ). If the cell is permeable to the solute, the cell must expend metabolic energy continuously to maintain a higher intracellular than extracellular concentration. Finally, the hydrolysis of proteins and nucleic acid to their constituent monomers and resynthesis of the macromolecules from those monomers undoubtedly consumes ATP. McGrew and Mallette (1962) have reviewed the opinions of microbiologists concerning the need for metabolic energy to meet the demand of chemical and physical wear and tear and have defined the energy required to maintain the status quo as the energy of maintenance. For the purpose of this paper the definition of energy of maintenance proposed by McGrew and Mallette will be modified such that the definition coincides with an experimentally measurable quantity, the specific maintenance (see equation 2 ) . The specific maintenance represents the consumption of the source of carbon and energy for purposes that are not a function of the rate of growth.THEORY The final crop of cells in a synthetic medium is linearly related to the concentration of an essential nutrient if that nutrient is made limiting ( Monod, 1942;Hinshelwood, 1946). It has been assumed that the yield of cells in a culture limited by the concentration of one nutrient is not a function of the rate of growth (Monod, 1942). The following equation is a consequence of that assumption:in which x is the concentration of cells; s is the concentration of the limiting nutrient; t is the time, and Y is the yield coefficient or the weight of cells produced per unit weight of nutrient.Some reactions might be expected to require an expenditure of energy per cell per unit time that is independent of the rate of growth. Such a requirement for energy would demand a diversion of part of the carbon source. This may be stated as a modification of equation 1.-* Present address:

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