Oxygen consumed by a living organism in aerobic reactions is used in adding mass to the organism and/or the production of chemicals and carbon dioxide. Since we may not know the molecular compositions of all species in such a reaction, it is common to define the ratio of moles of CO₂ produced per mole of O₂ consumed as the respiratory quotient, RQ, where 

Since it generally is impossible to predict values of RQ, they must be determined from operating data. Mammalian cells are used in a bioreactor to convert glucose to glutamic acid by the reaction 

The feed to the bioreactor comprises 1.00x10² mol C₆H₁₂O₆ /day, 1.20x10² mol NH₃ /day, and 1.10x10² mol O₂/day. Data on the system show that RQ = 0.45 mol CO₂ produced/mol O₂ consumed.

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nco2 noz nco2 or RQ %D no2