| | Red blood cells are placed into an isotonic watery solution. Nothing should happen. If we now introduce a solute in the bath that can cross the cell membranes (Fig. 1), the cells shrink rapidly at first by about 10%. Then they slowly regain their original volumes (Fig. 2; data by Macey and Oster, UBerkeley).
Interpretation The added solute raises the concentration of dissolved substances in the bath relative to the inside of the cells. Two things happen: The solute will travel into the cell (diffusion), and water will flow out of the cell into the bath (osmosis). Since only the amount of water in a cell effectively determines its volume, the volume of the cells shrinks (Fig. 2). At a certain point, the flow of water stops (the chemical potentials of water inside and outside have become equal). However, the permeable solute continues to diffuse (it only reacts to gradients of its own species, not to total dissolved substance like water). Since the concentration of solutes continues to grow inside the cells, water changes its direction of flow. The cells grow (Fig. 2).
(Experiment and data by Macy and Oster, UC Berkeley.) | 
Investigation 11 |
