Systems and surroundings

We need to understand the unique and precise vocabulary of thermodynamics before applying it to the study of bioenergetics. In thermodynamics, a system is the part of the world in which we have a special interest. The surroundings are where we make our observations (Fig. 1.1).

Fig. 1.1 The sample is the system of interest; the rest of the world is its surroundings. The surroundings are where observations are made on the system. They can often be modeled, as here, by a large water bath. The universe consists of the system and surroundings.

 The surroundings, which can be modeled as a large water bath, remain at constant temperature regardless of how much energy flows into or out of them. They are so huge that they also have either constant volume or constant pressure regardless of any changes that take place to the system. Thus, even though the system might expand, the surroundings remain effectively the same size.

We need to distinguish three types of system (Fig. 1.2): An open system can exchange both energy and matter with its surroundings and hence can undergo changes of composition. A closed system is a system that can exchange energy but not matter with its surroundings. An isolated system is a system that can exchange neither matter nor energy with its surroundings.

An example of an open system is a flask that is not stoppered and to which various substances can be added. A biological cell is an open system because nutrients and waste can pass through the cell wall. You and I are open systems: we ingest, respire, perspire, and excrete. An example of a closed system is a stoppered flask: energy can be exchanged with the contents of the flask because the walls may be able to conduct heat. An example of an isolated system is a sealed flask that is thermally, mechanically, and electrically insulated from its surroundings.

Fig. 1.2 A system is open if it can exchange energy and matter with its surroundings, closed if it can exchange energy but not matter, and isolated if it can exchange neither energy nor matter.