Osmosis is the movement of water from an area of high water potential to a low water potential. The blood plasma around red blood cells, for example must have a water potential the same as the inside of the cell so that there is no net movement into or out of the cell. Osmosis is also important because it can cause disease. For example, CF is caused by a defect in the CFTR gene which causes osmotic imbalance leading to mucus becoming much thicker than usual.