Water is a charged or polar molecule (H+ - O- - H+) that always moves across cell membranes. Scientists think this is possible because it is a very small molecule or because there are special spaces or pores that allow water to flow 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. Water molecules will show further movement from an area of ​​higher water concentration (and lower solutes) to an area of ​​lower water concentration (and higher solutes). In other words, net water flow tends to dilute an area of ​​higher solute concentration. When water moves by diffusion through a semi-permeable membrane, it is called osmosis. This is a type of passive transport because no 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" = identical, tonicity = tone or tension] or iso-osmotic for a cell if the cell has neither net gain nor loss of water. This is a dynamic balance. The cell and the extracellular solution have the same concentration of water and the same concentration of solutes. Our extracellular fluids must remain isotonic for cells to survive. If cells are placed in a solution containing a higher concentration of solutes than the cell, the cells experience a net loss of water and appear crenellated or wrinkled. These cells are in a hypertonic or hyperosmotic solution. Cells in a highly hypertonic solution may die. A solution whose solute concentration is lower than that present in the cells is said to be hypotonic or hypo-osmotic. In this case, excess water flows into the cells and they swell. These cells can eventually rupture or burst. This process is called lysis. Although we simplify osmolarity problems by using the % of a solute to