Before we take a look at how osmosis and tonicity affect a cell, let’s review what each of these terms means. Osmosis represents the diffusion of water across a semi-permeable membrane. The term tonicity refers to the relative solute concentration of two environments separated by a semipermeable membrane. In other words, by comparing the tonicity of the solution, you can determine the direction in which osmosis will occur. To demonstrate how tonicity affects a cell, let’s place some red blood cells into a beaker containing pure water. In this case, the solute concentration is greater inside the cells, than in the surrounding water. In other words, the contents of the cells are hypertonic in relation to the hypotonic contents of the beaker. Because of this, osmotic pressure results in the diffusion of water across the membrane and into the cells. Over time, if enough water enters the cells, the cell membranes may burst. This is called lysis. Now lets place the red blood cells in a beaker containing a solution of salt, such as sodium chloride. Since the contents of the beaker beaker are hypertonic in relation to the interior of the cell, the water within the cell will diffuse across the membrane and into the contents of the beaker. This causes crenation or shrinking of the cells. If the red blood cells are placed in a beaker whose contents match the tonicity within the cells, then there is no net gain or loss of water. The environments within the beaker and inside the cells are said to be isotonic, or the same. An important thing to remember, is that osmotic pressure always causes water to move from a hypotonic environment toward a hypertonic environment. In other words, water moves toward areas of high salt or sugar concentrations. This simple process is used to drive the operation of our kidneys and explains some of the physiological consequences of diseases such as diabetes.