Engineers use a number of types of boundaries when defining systems.  For instance, a boundary can be completely porous to material and energy so that an open system is defined in which energy and mass can flow in and out.  Alternatively, a boundary can be impervious to material but permit energy transfers in which case the system is term a closed system; and when the boundary does not allow energy or mass transfers then an isolated (& closed) system is defined.

In a recent thermodynamics lecture I explained how systems are defined by the type of boundaries used to describe them and gave an engineering example, an everyday example and a biological example for each type of system. So for an open system: a jet engine, a cup of coffee and the human body; for a closed system: a vehicle braking system, a water bottle and an egg; and for an insulated system: a refrigerator with the door closed, a sealed flask of coffee and I could not think of an insulated naturally-occurring biological system.

I challenged my students to identify an insulated system found in biology.  So far none of them have suggested any; however, one of them has challenged my example of an egg as a closed system.  He cited a web article from Scientific American [http://www.scientificamerican.com/article.cfm?id=bring-science-home-chick-breathe-inside-shell] to point out that the developing chick inside an egg has to breathe and hence gas molecules must cross the boundary thus making an egg an open system.  Perhaps, one could claim the soft-boiled egg that you have with toasted soldiers of bread is a closed system?  Are there any closed systems in biology?