Regulation of Cell Volume

The most abundant molecules both inside the cell and in the extracellular solution are water molecules, which make the major contribution to the volume of the intracellular and extracellular solutions. As a consequence, the flow of water into or out of the cell across the cell membrane is the primary determinant of changes in cell volume.

Water Channels

The cell membrane is highly permeant to water molecules. For much of the history of cellular physiology the high permeability of the cell membrane to water was something of a mystery because H₂O is a highly polar molecule that cannot easily cross the lipid bilayer. It was ultimately determined that there are water channels (or aquaporins) that selectively mediate the movement of water molecules across the membrane.

Water channels are integral membrane proteins, analogous to ion channels, that provide a low resistance pathway for the movement of water molecules across the cell membrane.

Osmolarity

Osmolarity is a measure of the concentration of osmotically active particles in a solution, typically expressed as osmoles of solute per liter of solution. For molecules such as glucose, sucrose and urea that do not dissociate, a solution containing 1 mole of dissolved molecules in 1 liter of water is a 1 osmole/liter solution. For salts or acids dissolved in solution the situation is slightly more complex because these compounds dissociate into two or more ions in solution. For NaCl, which dissociates into two dissolved particles, the Na⁺ and Cl^- ions, a 1M NaCl solution is a 2 osmole/l solution. For CaCl₂, which dissociates into three ions, a 1M CaCl₂ solution is a 3 osmole/l solution.

Regulation of Cell Volume

Maintaining a balance between the osmolarity inside the cell versus the osmolarity of the extracellular solution is critical to maintain the integrity of the cell membrane. To limit potential damage to the cell membrane, the osmolarity of extracellular solution is kept within relatively tight limits, in the range 275-295 mosmole/l in mammals.

To understand the effect of changes in intracellular or extracellular osmolarity on cell volume it is important to recognize that water has a concentration (number of molecules per unit volume) just like the solutes dissolved in a solution. The concentration of H₂O molecules in pure water is approximately 55.5M. If sugar molecules are dissolved into water the volume of the resulting solution increases because the sugar molecules take up some volume in the solution. Assuming that each solute molecule takes up the space of one water molecule, for a 1 M glucose solution, the water concentration falls to approximately 54.5M, significantly less that the 55.5M value for pure water. As a consequence, the concentration of water molecules in a sugar solution is lower than it is in pure water.

Water can flow down its concentration gradient across the cell membrane, just like membrane permeable solutes. If the concentration of water outside of a cell is higher than it is inside the cell, water will flow into the cell until the concentration of water is equal on each side of the membrane. An extreme example of this is if a cell is placed in distilled water (water containing no ions or other solvents). In this case the cell rapidly expands and dies, because the osmolarity inside the cell is much higher than outside and water flows rapidly into the cell, down its concentration gradient.