SUMMARY Increases in turbidity of strain K12 due to added non-permeant salts (NaCl and MgCl) and sucrose are strictly dependent on medium osmotic pressure, when correction is made for changes in medium refractive index. The volume of the whole cell and the fraction of the intact cell bounded by the cyto-plasmic membrane have been measured by dextran and [C]sucrose exclusion spaces. Increases in medium osmotic pressure due to non-penetrant medium solutes cause outflow of water across the cytoplasmic membrane and contraction away from the cell wall (plasmolysis), corresponding to the increases in turbidity. In addition salts (NaCl and MgCl) cause appreciable contraction in volume of the whole cell, presumably due to ionic interaction with the wall; sucrose causes only marginal decreases in whole cell volume. Electron micrographs of cells plasmo-lysed by NaCl or MgCl, but not by sucrose, show numerous adhesion points between the wall and the cytoplasmic membrane.

Glycerol penetrates the cell to the same extent as water, but because of its slower rate of penetration, transient decreases in volume occur which can be measured in a stopped-flow spectrophotometer due to concomitant increases in turbidity. In cells grown on glucose-containing medium the rate of glycerol penetration is non-saturating. In cells grown on glycerol an additional saturating-facilitated diffusion system is induced. Mutants deleted in the facilitator (F-) are available and do not show facilitated diffusion when grown on glycerol. Water exit and glycerol penetration in glucose-grown cells show transition points in Arrhenius plots corresponding to phase changes of the membrane lipids.


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