A theory explaining coordination of cell cycle events is alternation of the periods of exotrophy and endotrophy in a cell cycle (Ivanov, 2006). Cell cycle comprises the phases of exotrophy, when the external source of carbon and energy is extensively transformed into energy and carbon store (glycogen, starch, lipids), and the phases of endotrophy, when the accumulated store of energy and carbon is utilized for DNA replication and mytosis. External sources of energy and carbon are not assimilated during endotrophy periods. The alternations between the periods of exotrophy and endotrophy are performed due to the changes of intracellular concentration of cyclic AMP and are accompanied by alternation of the charge of membrane potential. Environmental factors which are unfavorable for DNA replication retain cells in phases of endotrophy. An extended period of exotrophy leads to enormous intracellular accumulation of carbon and energy sources. Exotrophic and endotrophic cells are distinguished by their biochemical and physiological properties so greatly that it would be useful to study these two different groups of cells. Therefore, distribution and percentage of exotrophic and endotrophic cells in the granule is an important information on its state and structure. For example, duration of exotrophy (Atex) of yeasts is linearly related to the duration of cell cycle (T): Atex = 0.5T — 1.0. Using this equation, the specific growth rate of the selected species in the granules (/x) can be determined from the microscopic view, taking into account that T = ln 2/^. Exotrophic and endotrophic cells can be distinguished after adding a small quantity of co-oxidizing substrate, which is transformed into toxic products of oxidation. For example, allyl or amyl alcohol can be added to cells, which utilize ethanol. As a result, cells will produce allyl or amyl aldehyde, which cannot be further oxidized and, therefore, will kill cells. Exotrophic cells die after this incubation but endotrophic cells remain alive because they do not consume and oxidize external sources of carbon and energy. The share of exotrophic cells increases during starvation and other unfavorable conditions because the phase of DNA synthesis cannot be started until sufficient intracellular quantities of carbon and energy sources are accumulated. However, distribution and percentage of exotrophic and endotrophic microbial cells in the granule, was not studied yet.
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