Osmotic Balance and Cell Volume

Figure 3-3 A simple model cell containing organic molecules, P. The ECF is a solution of solute, S, in water. Both water and S can cross the cell membrane, but P cannot.

Return now to the hypothetical primitive cell, early after the development of a cell membrane. In order for the cell membrane to do its job, it must be impermeable to the organic molecules inside the cell. But if the compositions of the extracellular and intracellular fluids are the same, with the exception of the internal organic molecules, the cell faces an imbalance of water on the two sides of the membrane. This situation is shown schematically in Figure 3-3. Here, the solutes that are in common in ICF and ECF are grouped together and symbolized by S. The extra solute inside the cell the organic molecules (symbolized by P, for protein) cause the concentration of water inside the cell to be less than it is outside. Put another way, the total osmolarity inside the cell is greater than it is outside the cell. There are two solutes inside, S and P, and only one outside. Water will therefore enter the cell and will continue to enter until the osmolarity on the two sides of the membrane is the same. Because the volume of the sea is essentially infinite relative to the volume of a cell and can thus be treated as constant, this end point could be reached only when the internal concentration of organic solutes is zero. This would require the volume of the cell to be infinite. Real cell membranes are not infinitely elastic, and thus water will enter the cell, causing it to swell, until the membrane ruptures and the cell bursts.

It will be convenient to summarize this situation in equation form. If a substance is at diffusion equilibrium across a cell membrane, there is no net movement of that substance across the membrane. For any solute, S, that can cross the cell membrane, this diffusion equilibrium will be reached when

The square brackets indicate the concentration of a substance, and the subscripts i and o refer to the inside and outside of the cell. Thus, in order for water to be at equilibrium, we would expect that

which is the same as saying that at equilibrium, the total osmolarity inside the cell must be the same as the total osmolarity outside the cell. For the cell of Figure 3-3, diffusion equilibrium will be reached only when the concentrations of all substances that can cross the membrane (in this case, S and water) are the same inside and outside the cell. This would require that Equations (3-1) and (3-2) be true simultaneously, which can occur only if[P] is zero.

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