Introduction
Most cells are freely permeable to water, which diffuses through aquaporins (water channels) in the plasma membrane. Water moves across the plasma membrane in response to the osmolarity (concentration of osmotically active substance in a solution) of the extracellular environment (Berne & Levy 1998).
Consequently, the osmolarity of the extracelluar environment can change the volume of a cell. If a cell is placed in a solution of higher osmolarity than its internal environment, the cell will shrink as water moves out of the cell. Conversely, if a cell is placed in a solution of lower osmolarity, the cell will swell as water moves into the cell. In an isosmotic solution the cell will retain its original volume as there will be no net movement of water.
The effect of a solution on the volume of the cells suspended in it is described as tonicity (Boron & Boulpaep, 2009). An isotonic solution does not alter the volume or shape of cells suspended in it because the osmolarity of the solution is the same as the cytosol of the cell. However, a hypertonic solution will cause the cells to decrease in volume and a hypotonic solution will cause the cells to swell.
The unique, biconcave shape of red blood cells (RBCs) in isotonic solutions enables them to change their volume over a much wider range of solution osmolarities than other cells. The volume of the RBC is inversely related to the osmolarity of the solution. RBCs placed in a hypertonic solution will lose water and shrink. This is termed crenation, where the shape of the RBC becomes shrivelled and spikey in appearance (Tortora & Derrickson, 2009). If the RBCs are placed in a hypotonic solution, they swell until they are spherical in shape. Further swelling will cause rupture of the RBCs and the loss of their contents (largely haemoglobin) into the surrounding solution (haemolysis).
Red blood cells were used as a model to investigate the physiology behind the movement of water across membranes. Red blood cells were placed in solutions of different osmolarities and tonicities and changes in cell volume were measured using a Light Intensity Meter (LIM) with Scope and Chart software.
