Systems and Surroundings

Thermodynamics in biology refers to the study of energy transfers that occur in molecules or collections of molecules.

When we are discussing thermodynamics, the particular item or collection of items that we’re interested in (which could be something as small as a cell, or as large as an ecosystem) is called the system, while everything that’s not included in the system we’ve defined is called the surroundings.

For instance, if you were heating a pot of water on the stove, the system might include the stove, pot, and water, while the surroundings would be everything else: the rest of the kitchen, house, neighborhood, country, planet, galaxy, and universe.

The decision of what to define as the system is arbitrary (up to the observer), and depending on what you wanted to study, you could equally well make just the water, or the entire house, part of the system.

The system and the surroundings together make up the universe.

There are three types of systems in thermodynamics: open, closed, and isolated.

• An open system can exchange both energy and matter with its surroundings. The stovetop example would be an open system, because heat and water vapor can be lost to the air.
• A closed system, on the other hand, can exchange only energy with its surroundings, not matter. If we put a very tightly fitting lid on the pot from the previous example, it would approximate a closed system.
• An isolated system is one that cannot exchange either matter or energy with its surroundings. A perfect isolated system is hard to come by, but an insulated drink cooler with a lid is conceptually similar to a true isolated system. The items inside can exchange energy with each other, which is why the drinks get cold and the ice melts a little, but they exchange very little energy (heat) with the outside environment.

[Why is a cooler sometimes called a “closed” system?]

If you’ve watched the video on the Second Law of Thermodynamics and entropy, you may have noticed that Sal refers to this same example, a cooler, as an approximation of a “closed” system. Here, it’s described as an approximation of an “isolated” system. What’s up with that?

As it turns out, this is a case where two different branches of physics use terms in slightly different ways.

• A system that doesn’t exchange either energy or matter with its environment is called an “isolated” system by physicists who study thermodynamics, but a “closed ” system by physicists who study classical mechanics. In his video, Sal uses the classical mechanics definition, while this article uses the thermodynamics definition.

You, like other organisms, are an open system. Whether you think about it or not, you are constantly exchanging energy and matter with your surroundings. For instance, suppose that you eat a carrot, or lift a bag of laundry onto a table, or simply breathe out and release carbon dioxide into the atmosphere. In each case, you are exchanging energy and matter with your environment.

Exchanges of energy that take place in living creatures must follow the laws of physics. In this regard, they are no different from energy transfers in, say, an electrical circuit. Let’s take a closer look at how the laws of thermodynamics (physical rules of energy transfer) apply to living beings like yourself.