What is meant by selectively permeable?

Selective permeability is a term used to describe cell membranes. Cell membranes that are selectively permeable only allow certain molecules to cross into and out of the cell. Molecules are able to pass through a selectively permeable membrane through active or passive transport mechanisms.

Passive transport allows small molecules to pass easily through the cell membrane. This mechanism can allow molecules to pass through the membrane via osmosis, simple diffusion or facilitated diffusion. Active transport allows larger molecules and proteins to pass through the cell membrane. This type of transport requires energy and occurs through protein pumps or vesicles within the cell membrane.