Timecourse of Volume Changes

So far in the discussion of maintenance of cell volume, we have considered only the final, equilibrium effect of a solution on cell volume and have ignored any transient effects that may occur. To see such transient effects, consider what happens to the model cell immediately after it is placed in the solution in Figure 3-4d, 0.25 M urea + 0.25 M sucrose. This is summarized in Figure 3-5. At the start, the osmolarity outside (0.5 Osm) is greater than the osmolarity inside (0.25 Osm), and water will initially leave the cell as it diffuses down its concentration gradient. Urea, however, begins to diffuse into the cell down its

Figure 3-5 Time-course of cell volume when the model cell is placed in the solution used in Figure 3-4d.

Figure 3-5 Time-course of cell volume when the model cell is placed in the solution used in Figure 3-4d.

concentration gradient. Thus, the internal osmolarity begins to rise as a result of the increasing [urea]i and the loss of intracellular water. The leakage of water out of the cell slows down and finally ceases altogether when [P]i + [urea] = 0.5 M; that is, at the point when internal and external osmolarities are equal. At this point, however, [P] is higher than its initial value (0.25 M) because of the reduction in cell volume, and [urea] is thus less than 0.25 M. Urea therefore continues to enter the cell to reach its own diffusion equilibrium, and the internal osmolarity rises above 0.5 Osm, so that water enters the cell and volume begins to increase. This situation continues until the final equilibrium state governed by Equations (3-4) and (3-6) is reached. What would you expect the time-course of cell volume to be if the model cell were placed in an infinite volume of a solution of 0.5 M urea?

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