Carbon:energy-balanced substrate

Carbon from the substrate glucose is converted into the carbon of the cells, phenylalanine, carbon dioxide and byproducts. Carbon balance calculations thus give us more understanding of the amount of carbon in glucose used for cell mass production, for synthesis of the wanted produd, maintenance energy and byproduct formation. 

Gross measurement errors are easier to detect when several elemental balances (for carbon, nitrogen, degree of reduction) can be checked. Because heat (enthalpy) is an extensive variable, an energy balance can be set-up around the reactor boundary considered as an open system. Therefore heat can be used to check that no major substrate or product has been neglected. 

The characterization of energy balance in cells and tissues relies upon the measurement of several variables oxygen and substrate consumption, lactate and carbon dioxide production. These measurements however give only indirectly and partly quantitative information about the energy balance in obesity. Whole body calorimetry and direct measurement of cell heat production have been introduced as a complement to the assessment of energy balance in the whole organism and energy expenditure on the cellular level. 

Turning to the substrate balance, yeast cells contain about 50% carbon. The cell mass is measured as total dry weight, not just carbon. This gives Yx/s = 2 when S is measured as the carbon equivalent of glucose. A reasonable value for Yxis is 1 so that half the carbon goes into biomass and half meets the associated energy requirements. The maintenance coefficient in carbon-equivalent units is 0.008 h . Using these parameter estimates, the three simultaneous ODEs for 5" > 0, become... 

Exopolysaccharide production ntay be improved by tiie provision of various organic con xments, otiier Aan the main carbon and energy source. These can improve growth of the production organism (growth factors) and/or directly enhance the synthesis of exopolysaccharide. Additions that improve polymer yield include tricarboxylic add (TCA) cyde intermediates, which are thou t to improve metabolic balance between carbon flow frcnn carbohydrate substrate through the catabolic pathwaj arul oxidation through the TCA cyde. 

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