The physical properties and photosynthetic metabolism of synchronized cells ofChlorella pyrenoidosaare described. Cells, synchronized by successive periods of light and dark, photosynthesized under steady-state conditions for 30 min with14CO2. Pool sizes of metabolic intermediate compounds, and rates of flow of carbon through these pools, were determined. Cell properties and metabolism were studied for cells just divided, at three periods during the growing stage, at the time of maximum DNA synthesis, just prior to division (after continuous light, and after 5 hr darkness), and following division after continuous light for 37 hr.Changes in pool sizes and flow rates are correlated with relative shifts between amino acid and protein synthesis, which is greatest during the growing period and DNA-synthesizing stage, and sucrose synthesis, which is greatest in the divided cells (after darkness) and pre-division cells (after darkness). The effects of the stage of cell growth and of a prior period of darkness can be separated to some extent by these studies, and in some respects are additive.Specific sites of metabolic regulation discerned in these experiments include the following: 1) Ribulose diphosphate carboxylase, 2) fructose diphosphatase and sedoheptulose diphosphatase, 3) the synthesis of sucrose, probably at the reaction between fructose-6-phosphate and uridine diphosphoglucose to give sucrose phosphate and uridine diphosphate, 4) amino acid synthesis, at the level of nitrate reduction, and 5) amino acid synthesis, at the level of carbon flow from the photosynthetic carbon reduction cycle to amino acid carbon skeletons.
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