Analysis of transcellular water movement inNitella

Summary The mechanism of transcellular osmosis was analyzed on the assumption that the driving force, which is equal to the osmotic pressure of the mannitol solution given to the exosmosis side, is divided into two parts; one causing the inward water flow on the water side, the other causing the outward water flow on the solution side, when each force drives an equal amount of water. Based on this analysis a new procedure was developed to measure the endosmotic and exosmotic water permeabilities of the membranes independently. It involved measurement of volume of water transported transcellularly, change in turgor pressure, and water permeability of the cell wall alone.Experiments following the new procedure revealed that in aNitella internode positioned across a partition wall with equal length both the endosmotic and exosmotic water permeabilities remained constant during transcellular osmosis induced with 0.4M mannitol, at least for the first minute. It was found that the permeability coefficient for endosmosis (3.9 × 10^–5 cm sec^–1 atm^–1) was very much higher than that for exosmosis (1.4× 10^–5 cm sec^–1 atm^–1). Treatment of the endosmotic cell part with 5% ethanol conspicuously decreased the water permeability of the cell on this side down to 1/2.4 the value obtained without ethanol but never affected the permeability on the other side (exosmosis side).This work was supported partly by a Research Grant from the Ministry of Education of Japan.