A selectively permeable membrane
is a thin barrier
which allows passage of some substances
through it, but prohibits the transport
of others, based on specific criteria
, such as size
(or lack thereof) for certain substances, structure, or presence of chemical bonds. Osmosis
is an example of passive transport
over a selectively permeable membrane, and filtration
is an example of active transport
over a selectively permeable membrane.
Selectively permeable membranes are found throughout the field of biology: the cell's plasma membrane is one, one's intestines are another, there are such membranes in the lungs which allow exchange of oxygen and carbon dioxide with the blood. Mechanical filters usually operate on this principle. Think of a selectively permeable membrane as a wall with holes in it. Small stuff may be able to go right through it, while big stuff has no chance. There might be other factors affecting what stuff goes through: a square peg might not go through a membrane with round holes.
See osmosis for an interesting demonstration of selectively permeable membranes.