Maintenance energy flux

Maintenance energy flux in non-growing/starved cells is not easily studied. There are at least two difficulties (/) the low rate of metabolic flux and (//) the complexity of potential intracellular energy sources. Microcalorimetry may be an appropriate technique for studies of maintenance energy flux, both because it is not restricted to the metabolsm of any particular substrate and it is a relatively sensitive method (see also section 2.2. for a review see [4]). Care must be taken, however, in how these experiments are performed. It is quite common to concentrate and/or transfer the cells to a new (starvation) medium for these types of determinations. At least in yeast such a transfer has been shown to trigger some kind of endogenous metabolism [26], which should not be confused with maintenance metabolism since periods of an increased 

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Heat flux calorimeters are bioreactors equipped with special temperature control tools. They provide a sensitivity which is approximately two orders of magnitude better than that of microcalorimeters, e.g. [33,258]. The evaluation and description of microbial heat release is based on a heat balance heat yields and the heat of combustion of biological components are central parameters for quantification [70]. Measurements obtained so far have been used to investigate growth, biomass yield, maintenance energy, the role of the reduction degree of substrates, oxygen uptake [414] and product formation [272]. 

The purine nucleotide cycle of muscle consists of the conversion of AMP —> IMP AMP and requires AMP deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase (Figure 27-24). Flux through this cycle increases during exercise. Several mechanisms have been proposed to explain how the increase in flux is responsible for the maintenance of appropriate energy levels during exercise (Chapter 21). 

Precoat Vacuum Filter (Petrides, D., MIT, personal comm., 1988) Flux 200 l/m -hr 0.1 kg diatomaceous earth per kg solid removed Cake washwater sprayed on drum at a rate of 30% of feed flowrate gives 90% yield Energy requirements 0.12 Kw/m Capital cost correlation Labor 2 man-hrs/shift at 20/hr Maintenance 5% of capital Installation cost 70% of Purchased Price Indirect costs 30% of Purchased Price... 

Several cost functions or criteria of excellence can be proposed for this problem, depending on whether some use can be made of the intermediate flux (which could consist of a limited period at full power between periods of zero power) or the need to allow activity to die away before maintenance work is undertaken, etc. For the purpose of the example here, we shall consider two cost functions minimum time and minimum lost flux-time. In the first, the control period itself is to be kept as short as possible in the second, the criterion is based on the wasted potential for delivering nuclear energy. These two examples are representative problems. Both, of course, have a considerable literature, having been approached by more than just the Pontryagin method. Our treatment follows that of Roberts and Smith (25,26) for the time optimum problem. 

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