Water Permeability of a Characean Internodal Cell with Special Reference to Its Polarity

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Abstract
Water permeability (hydraulic conductivity) of the Nitella inter node was studied by means of transcellular osmosis with special reference to its polarity. It was shown that the change in turgor or in cell volume involved in transcellular osmosis is almost over within 5 sec after the onset of both forward and backward osmoses. What is different with respect to turgor between the two osmoses is its final level. In forward osmosis the turgor drops down to a definite level depending on the external concentration, while in backward osmosis the turgor invariably comes back to the normal level. "Transcellular osmotic constant" (K) and "transcellular permeability constant to water" (K), both of which are defined in the present paper, were determined by the rate of water flow measured at the initial phase of transcellular osmosis after the turgor change has been almost accomplished. Osmosis experiments were performed using the cell wall tube freed from all the cell contents. Exosmosis induced by a mannitol solution outside the cell wall tube occurs almost linearly with osmotic concentration of the mannitol. Exosmosis induced both by the external sucrose solution and simultaneously by the internal hydraulic pressure has a rate that is just equal to the sum of the rates of each osmosis induced separately. This shows that the exosmosis caused by the external sucrose solution is not influenced by the outflow of water hydraulically induced. The value of the polarity (p), or the ratio of endosmotic (ken) to exosmotic (keX) water permeability constants, varied from cell to cell, values ranging from 1.2 to 2.7 with an average of 1.7. There is, however, hardly any correlation between p and the ratio in which the cell is partitioned. The facts summarized in the preceding paragraphs show that the polarity is not explained in terms of the dilution effect ("sweeping-away effect"). We thus conclude that polar permeability is an intrinsic characteristic of the living cell. Transcellular permeability constant to water (K[image]) decreases with the increase in osmotic concentration of the external medium in forward osmosis, but not in backward osmosis. The decrease in K[image] in forward osmosis is more conspicuous the less the turgor level. When the osmotic concentration of the cell sap is made higher than normal, K[image] decreases, and when it is brought to a subnormal level, k[image] increases. The effects of turgor and of the osmotic concentration of cell sap on water permeability may be explained in terms of hydration and dehydration on the part of the cytoplasm, including membranes.

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