ABSTRACT
A living organism can survive only by exchanging materials with its environment, that is, by being an open system. It takes in materials such as food or sunshine or oxygen, and transforms these into what is required for survival, excreting what is not used as waste. This requires certain properties, notably an external boundary, a membrane or skin which serves to separate what is inside from what is outside, and across which these exchanges can occur. This boundary must be solid enough to prevent leakage and to protect the organism from disintegrating, but permeable enough to allow the flow of materials in both directions. If the boundary becomes impermeable, the organism becomes a closed system and it will die. Furthermore, exchanges with the environment need to be regulated in some way, so that only certain materials enter, and only certain others leave to return to the outer environment.
