Semi-permeable membrane

 A Semi-permeable membrane is one of the different types of biological or artificial membranes which only allows certain molecules or ions to pass through it. Artificial membranes such as those used in reverse osmosis, are designed with a variety of materials for the purpose of filtration and they only allow water to pass through. The Biological membranes of the cells however are found in a phospholipid bilayer form. Water and other such smaller molecules pass through the gaps in the bilayer but larger molecules or ions cannot force their way in or out of the cell.