Re: why do cells have celectively permeable membrane?

Selectively permeable means that only specific molecules can cross the plasma membrane.
This is a very important characteristic of cellular membranes. Inside each cell is a specific 
combination of water, salt, sugars, nucleic acids, fats, and proteins. If that combination is 
disrupted, the cell won't work properly. Sometimes it can even die. For example, if too much 
water crosses the plasma membrane, the cell will lyse, or burst apart from the pressure. 
Alternatively, if too much salt crosses the membrane, there won't be enough liquid inside 
the cell for it to perform its functions. 

For some proteins, specific movement of materials across the membrane is required for 
function. For example, when a nerve cell wants to send information to another cell, 
the rapid transport of Sodium and Potassium across it's membrane results in an electrical 
current that passes through the axon of the cell and causes the release of chemical 
information that is picked up by another cell.  Without selective permeability, this electrical 
current could not be achieved, and our nerves wouldn't work.