permeable and semi permeable membranes?

[rosieburk]

permeable and semi permeable membranes? What's the difference? I've tried searching the net but theres a lot on semi permeable membranes but not that much on plain permeable membranes.

[canalon]

Because it does not really make sense... A permeable mebrane is a membrane that let everything go through. But for example even if cheesecloth would be quite close to this definition, it is impermeable to say lephants or bowling balls (at least the slow kind). So the notion of semi permeability is defined relatively to a class of objects, for exemple molecules with varying size.