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Clearly, the results of metabolism may be reflected by simultaneous changes in the concentrations of several classes of macromolecules as well as the water content of the cell. Lagunas and Moreno (1985) pointed out that intracellular volume, cell dry weight, protein content and cell number change independently of each other. For these reasons direct comparison of published data can be difficult.

Ogur et al. (1952) described the RNA and DNA contents of a laboratory S. cerevisiae cells as influenced by ploidy. In this case, the results were expressed as a function of cell number (Table 4.5). Similar proportionalities between RNA and DNA can be observed in brewing yeast (D.E. Quain and P.A. Thurston, unpublished data). In this case the contents of RNA, DNA, glycogen and protein content of brewing yeast were monitored, together with dry weight and cell number, during fermentation an all-malt wort. Cell number increased during fermentation to a maximum value of approximately 80 x 106 per ml. This was accompanied by an increase in dry biomass. The latter parameter continued to increase after the cessation of cell proliferation from 4.7 to 6.0mgml 1. RNA and DNA increased in parallel with cell number. The concentration of RNA, expressed as a function of cell dry weight, was approximately 5%. This amounted to 50 times the concentration of DNA. The protein concentration increased in a linear fashion throughout fermentation, eventually accounting for approximately 30% of the cell dry weight. The increase in biomass observed in the stationary phase, when cell proliferation had ceased, was a result of the accumulation of glycogen. In the subsequent maintenance phase prior to removal from fermenter, the dried biomass decreased and this was in large part due to glycogen dissimilation (see Section 3.4.2.1).

Table 4.5 Nucleic acid content of Saccharomyces cerevisiae in cells of different ploidy (from Ogur et al., 1952).

Ploidy

RNA (ng/cell x 109)

DNA ((ag/cell x 109)

Haploid

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