Movement of Water Against a Gradient in Models

Abstract

In certain models employed to imitate living cells the behavior of water appears to be the opposite of what is expected. If it acts similarly in vivo it is not hard to understand why the living cell often fails to behave as a perfect osmometer.

In these models water moves through liquid layers against a gradient, passing from a region where its activity is low to one where its activity is high. This depends upon the fact that certain substances traversing these layers appear to carry water with them.

The simplest form of model is made as follows: In a U-tube (Fig. 1) we place guaiacol∗ (B) to imitate the action of the protoplasm. 1 Resting on the guaiacol (which forms a separate phase in contact with water) at the left is an aqueous solution (A) representing the external solution, and at the right an aqueous solution (C) representing the cell sap.

We may begin by putting in B some guaiacol which has been shaken with distilled water until equilibrium has been attained: in A and C we place equal amounts of the water which has been shaken with guaiacol. The system is therefore in equilibrium throughout and no movement of water occurs.

When we lower the activity of the water in A by adding trichloroacetic acid we might expect water to move from C to A, but just the opposite occurs. Acid and water move from A to B and from B to C. The higher the concentration of acid in A the greater the movement of water into C.