Substrate balance

Lipogenesis from carbohydrates in humans is not a very common metabolic situation. It would require observed RQ-values to be higher than 1. This is sometimes seen, but almost always within the range between 1.00 and 1.05. This could also be interpreted as measurement errors or noisy results. Lipogenesis would require a temporary oversupply of carbohydrates, that could not be absorbed into the glycogen stores. This situation usually happens within some h after a big meal. It also requires a relatively low energy expenditure like in 

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For the substrate balance, it is known from Equation 11-61 that... 

The substrate balance in a batch culture for component i in the culture volume of VR and change of molar concentration of C, is equal to the rate of formation of product ... 

The substrate balance is given based on following equation ... 

Substrate balance may also lead to the same results as the following relations ... 

For the special case when /x = D, the substrate balance equation reduces to yield of substrate to cell biomass ... 

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... 

BIOMASS BALANCE SUBSTRATE BALANCE PRODUCT BALANCE... 

The total substrate balance for the whole fermentor is given as ... 

We can obtain a model, i.e., the design equations for the digester process, by carrying out organism and substrate balances and using the following relationships ... 

The possibility of indirectly measuring cell biomass and growth rate by computer-aided on-line substrate balancing was first mentioned in a publication by Humphrey (11). The method is discussed in detail in a review by Humphrey (1). The technique operates by noting that the difference in amount of a particular component in and out of the vessel is equal to that used by the organism for growth and maintenance if no appreciable secondary metabolites are formed. Mathematically this can be stated as... 

For cells in the stationary phase, where there is no growth, cell maintenance and product formation are the only reactions to consume the substrate. Under these conditions the substrate balance, Equation (7-118), reduces to... 

In a number of cases extra attention must be paid to the substrate balance. If product is produced during the growth phase, it may not be possible to separate out the amount of substrate consumed for growth from that consumed to produce the product Uiider these circumstances all the substrate consumed is lumped into the stoichiametric coefficient, and the rate of substrate disappearance is... 

The substrate concentration, S to use in Equation 20 can be determined from the substrate balance (Equation 8). To calculate Z, it is necessary to make a material balance as shown in Equation 21. 

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