Transcellular Osmosis and Polar Water Permeability

In April 1952,1 started experiments to measure transcellular water transport in the internodal cells of Nitella by using the so-called double chamber osmometer (Kamiya and Tazawa 1956). To perform transcellular osmosis (TCO), an internodal cell is partitioned into two chambers A and B. When water in B is replaced with a sucrose solution, water moves transcellularly from chamber A to chamber B. The rate of the flow is proportional to the osmotic pressure (po) in B. The TCO constant (K) defined by Kamiya and Tazawa (1956) is obtained by dividing the initial rate of water flow (Jv) by the external osmotic pressure (po) that drives the flow. Namely,

Jv/po = K = (LpenLpexAenAex )/(LpenAen + LpexAex), (1)

where Lp and A represent the hydraulic conductivity to either endosmosis (en) or exosmosis (ex) and the surface area of the cell part on either the endosmosis or the exosmosis side, respectively. If Lpen = Lpex = Lp and Aen = Aex = A/2, we get

where A represents the surface area of the whole cell and Lp in (2) represents the apparent hydraulic conductivity of the entire cell.

One day, Professor Kamiya suggested that I measure the hydraulic conductivity using an asymmetrical arrangement in order to test whether the water permeability might depend on the direction of osmosis. He certainly knew about the rectification property of the nerve fiber in terms of electric current. In axons, the electrical resistance to the depolarizing current is lower than it is to the hyperpolarizing one. Using an asymmetrical arrangement, to measure hydraulic resistance in characean cells, two reciprocal osmoses can be performed, one from the shorter half to the longer half and another in the reverse direction. These two reciprocal osmoses give two simultaneous equations corresponding to (1). Solving the equations, we get Lpen and Lpex.

We found that Lpen is larger than Lpex. Namely, water enters the cell easier than it escapes from the cell. The polar hydraulic conductivity was intensively studied for its cause by us in Osaka and by Dainty and coworkers in Edinburgh. In our first paper, the polar water permeability to water was attributed to an intrinsic characteristic of the plasma membrane (Kamiya and Tazawa 1956).

On the other hand, Dainty and Hope (1959) were of the view that the polar permeability to water was only apparent and was caused by a "sweeping away" of the solutes on the exosmosis side, thus lowering the effective concentration of the solute at the surface of the plasma membrane. In the asymmetrical arrangement, such a dilution-effect due to the sweeping away would be larger when the solution is given to the shorter cell part than when given to the longer cell part.

Later, Dainty and Ginzburg (1964a) found in Chara that hydraulic conductivity decreased markedly with an increase in the external sucrose concentration. They found that the inhibitory effect of the external osmolality cannot be attributed to the sweeping away effect. The inhibitory effect of the external osmotic pressure on hydraulic conductivity was reconfirmed by Kiyosawa and Tazawa (1972) in Nitella flexilis.

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