Water permeability of isolated protoplasts in relation to volume change
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Unusual accuracy in the measurement of the permeability of protoplasm to water was obtained by taking time-lapse photomicrographs of isolated protoplasts during osmotic passage of water.
The permeability of free protoplasts is the same as that of protoplasts enclosed in their cell-walls.
When determined at different stages of deplasmolysis (or deplasmorrhysis) the rate of penetration of water showed a gradual and relatively steady rise to a little beyond the normal size of the cell, and then a sharp jump upward.
Per contra, at different stages of plasmorrhysis the rate of penetration remains nearly constant with only a slight falling off in high concentrations of plasmolytic (9 NaCl ∶ 1 CaCl2).
At normal to 3/4 normal volume of the cell, water permeability (averaged from our results) is about two and a half times as great when determined by deplasmorrhysis as by plasmorrhysis, but the value of the former varies with the rate of distention of the cell. Below 40 % of normal volume, plasmolysis and deplasmolysis give the same results.
The rise in permeability with degree of deplasmorrhysis is primarily a mechanical effect of the extension of the membrane, an effect which increases with both degree and rate of stretching. Increased hydration is not important, or else its effect would show in plasmorrhysis.
When allowance is made for change in thickness of the protoplasmic layer, the calculated permeability tends to show increase rather than decrease with the degree of plasmorrhysis.
The normal rate of penetration of water into cells of onion pulp is about 20μ per hour at 20° C., under a pressure difference of 1 atm.
KeywordsCaCl2 Normal Rate Volume Change Pressure Difference Normal Volume
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