Most of us are so accustomed to the word "system" used in our everyday dialog, we rarely think about what it means. A formal definition might note that a system is a group of parts that come together, interacting and interdependent, to form a more complex whole unit. Sounds confusing, yes - a simpler and more familiar way to say this might be that the whole unit is greater than the sum of the parts. Before we look into the broad implications of this idea, and how it relates to "systems biology", let's look at a very simple system. Consider for a moment the following three items:

1. a small metal cup, about the size of a walnut.
2. a glass bowl, in the shape of a small balloon.
3. a length of tungsten wire, very brittle alas, pinched up into a small coil.

On their own, each of these parts is rather useless. But what happens if we put them all together? The wire coil could be tucked up inside the glass balloon; the metal cup could be attached to end of the glass balloon, sealing off the opening. This would allow us to create a vacuum inside the glass balloon. The metal cup could also act as electrical contacts, passing current to the wire coil. Sound familiar…?

Suddenly we have three relatively useless parts working together as a system: if you had never seen one of these before in your life, you might not guess what it does. But of course you know it as a light bulb. And you know it has this amazing ability to give us light --something that would be totally unexpected from just looking at each of the three oddball parts in our original list. And yet something which has profoundly changed our lives.

Of course this system we call a "light bulb" can itself become a part within larger systems. Combine light bulbs with plastic grass and wooden benches and suddenly you have evening soccer matches and major-league baseball games. Combine light bulbs with concrete pavement, flying machines and computerized schedules and suddenly you have round-the-clock flights that in turn give rise to overnight delivery services. The point is that individual parts, seemingly mundane on their own, can combine in unexpected ways into a "system". And the interaction of the parts in this system creates important properties or functions we would not expect from looking at the individual parts, each on their own.

Emergent Properties

We call these properties and functions that arise from the interacting parts in a system "emergent properties". The concept of emergent properties is central to the study of systems. Any function performed by a system that is not the result of a single part in the system, but rather is the result of interacting parts in the system, is an emergent property. The light bulb's ability to generate light cannot be attributed to any of the three parts in the system on their own. Rather, it is the result of the combination of the metal cup sealing the glass bulb and passing current to the wire coil, the glass bulb maintaining a vacuum and still allowing light to radiate out, and the wire coil glowing hot enough to give off light without melting itself or the glass balloon. The light bulb's ability to give off light emerges only as a result of the interactions of the metal base, the glass envelope, and the tungsten wire filament.

Systems are Irreducible

Systems that have emergent properties are said to be irreducible. They cannot be reduced to their individual parts or studied one part at a time, with the expectation of understanding the emergent properties of the system. Remember, emergent properties are the result of the interactions between system elements, not the result of parts on their own. Put another way, just as we could not hope to understand an author´s meaning by studying a book one word at a time, we cannot hope to understand and be able to predict the function of a complex system by dissecting it into individual parts, and studying each part on its own.

Complex Systems

A system is said to be complex if its emergent properties are unpredictable. Earlier, we mentioned overnight delivery services. We might understand the function and purpose of light bulbs, runways, airplanes, delivery trucks, bar codes, and even computerized schedules, on their own. But if we had never experienced such a system before, that knowledge alone would not enable us to readily predict these could combine into a system which provides efficient, overnight global delivery of tens of millions of packages.

Life itself is another example of a complex system. You might agree that life cannot be predicted simply by analyzing the chemicals and organic compounds that comprise the tissues and organs of which we are made. We certainly can´t mix a spade of dirt with a bucket of water and expect to get new life. Rather, we can only hope to fully understand life in all its complexity by studying the interaction of all the parts that comprise each organism.

With our growing understanding of the genetic constitution of all life forms, and how genes affect our form and function, modern biology increasingly focuses on how genes interact, along with how the chemical compounds (i.e., proteins) produced from genes also interact. The complexity of humankind´s genetic makeup (approximately 25,000 genes) plus the myriad of proteins produced from these genes, give rise to the extraordinary functions of human beings (remember, emergent properties), and the corresponding complexity of a human being as systems.

In summary, systems are comprised of parts which interact. The interaction of these parts gives rise to new properties and functions which are key to the system. We call these new properties and functions "emergent properties". Because emergent properties are the result of interactions between the parts, they can not be attributed to any single parts of the system. This makes systems irreducible. A system is unlikely to be fully understood by taking it apart and studying each part on its own. (We cannot understand an author's message by studying individual words; we cannot appreciate a forest by looking at individual trees.) To understand systems, and to be able to fully understand a system's emergent properties, systems need be studied as a whole. This recognition that complex systems, especially life, are truly understood from knowledge of the interactions of their component parts is fundamental to systems biology and all the research at the Institute for Systems Biology.

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