Water is a charged or polar molecule (H+ - O- - H+) that is always moving across cell membranes. Scientists theorize that this is possible because it is such a small molecule or because there are special spaces or pores that allow water to move across the cell membrane. The predominant direction of water flow is determined by the concentration of solutes (molecules other than water) inside and outside the cell. The water molecules will show a new movement from an area of ​​higher water concentration (and with fewer solutes) to an area of ​​lower water concentration (and with higher solute content). In other words, the flow of mains water tends to dilute an area with a higher concentration of solutes. When water moves by diffusion through a semipermeable membrane it is called osmosis. This is a type of passive transport because non-cellular energy (ATP) is involved in the movement of water. For convenience we will use tonicity and osmolarity as interchangeable terms. In fact, there are exceptions when these terms do not have the same meaning. An extracellular solution is isotonic ["iso" = same, tonicity = tone or tension] or iso-osmotic for a cell if the cell has no net gain or loss of water. This is a dynamic equilibrium. The cell and the extracellular solution have the same water concentration and the same solute concentration. Our extracellular fluids must remain isotonic for cells to survive. If cells are placed in a solution that contains a higher concentration of solutes than the cell, they experience a net loss of water and appear crenate ["cren" = notched] or wrinkled. These cells are found in a hypertonic or hyperosmotic solution. Cells in a highly hypertonic solution can die. A solution that has a solute concentration lower than that present in cells is said to be hypotonic or hypoosmotic. In this case, excess water flows into the cells and the cells swell. These cells may eventually rupture or burst. This process is called lysis. Although we simplify osmolarity problems by using percentage of solute