SUMMARY: An osmotic barrier, impermeable to inorganic phosphate ions, is present near the external surface of var. (strain Duncan). The barrier encloses a volume of 1·66 ±0·07 ml./g. dry-wt. cells, within which the inorganic phosphate of the cell is confined at a concentration of 0·1 m. The inorganic phosphate of the internal and external media exhibit a rapid reciprocal exchange, sensitive to traces of heavy metals. The existence of the osmotic barrier and the characteristics of the phosphate exchange are not in accord with an exchange-adsorption hypothesis; they are in accord with an exchange-diffusion hypothesis, according to which phosphate is shuttled across the osmotic barrier by carrier molecules. There is no experimental justification for supposing the acid-soluble inorganic phosphate of to exist as an acid-labile compound.


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