Osmosis is the process by which the concentrations of two solutions equalize across a selectively permeable membrane. Osmosis is necessary for the digestive and excretory systems to function. This demonstration models the behaviour of cells in hypertonic and hypotonic solutions.
The terms hypertonic and hypertonic are used in the context of a solution surrounding living cells. If a solution has a higher concentration of a solute than occurs within the cells, the solution is hypertonic. This causes water to move from the cells through the membrane into the hypertonic solution. A hypotonic solution is a solution with a lower concentration of solute than occurs within the cells. This causes water to move from the hypotonic solution through the membrane into the cells. An isotonic solution is a solution with the same concentration of solute as the cells. In an isotonic solution, therefore, the cells will neither gain nor lose water.