A membrane is a soft pliable sheet made from plant, animal, or a man-made Material.   Membranes can be described as permeable or semi permeable.   Semipermeable means that some materials can pass through the membrane but other materials cannot.    The osmosis experiment is a prominent example of a semipermeable membrane.   Water and wine were separated by a membrane from a pig’s bladder.   It appeared that the wine could not pass through the membrane but the water could.   Later analysis determined that the membrane could pass small molecules (water), but it could not pass the larger molecules in wine.    Others showed that a sugar water solution behaved just the same as wine.   The larger sugar molecules could not pass through the membrane but the water could.   Experiments used different solutes (smaller molecules).  Some solutes could pass through the membrane as well is water.   A semi permeable membrane has holes small enough for some molecules but too small for others.

Membranes are important in all aspects of our world.    In biology, cells are surrounded by a membrane.   Nutrients pass into the cell and waste products leave the cell through the membrane wall.     Our body organs are all protected or encapsulated by a membrane. Membranes provide a protective layer between a cell or a body organ and the other fluids in the body.   Our skin could be thought of a thick membrane that separates our body organs from the environment.   

Membranes are also important in construction.    Buildings use many membranes. A membrane under a concrete floor impeded moisture flow from the soil to the building interior.    Membranes are used in walls to reduce moisture transfer between the inside rooms and the outside environment.  Building walls reduce air flow, heat transfer and moisture transfer.  They also provide an opaque barrier between the building occupants and the outside environment.

Many buildings use a forced air system for heating and cooling. Filters in the air ducts remove dust, pollen, bacteria, and mold from the circulating air.   Water filters are used to provide safe drinking water.   Reverse osmosis systems provide an even cleaner water product. 

Oil and gasoline filters are used to protect engines from harm. 

In summary, membranes are important in biology and in man-made construction.   Our next article will examine flow mechanism through a membrane.