The Laws of Thermodynamics

Thermodynamics is concerned with the relationships between energy, heat, and work. With its origins in the empirical problem of how to make engines work efficiently, classical thermodynamics was fleshed out during the Age of Steam. This is why many of its units and concepts (the joule, the watt, the kelvin, the Carnot cycle) are named after the engineers and physicists that made steam power a practical tool.

It turns out, though, that the rules governing the energy relations of steam engines also govern any system—including living things—that does work of any sort. For example, one can express the work done by a steam engine as a function of the fuel energy put into it. One can do the same for an "organism engine," such as a harnessed horse, fed with hay, that works to drive a millstone. Interestingly, the work done by both engines will be constrained in some rather fundamental ways, suggesting that the two are governed by the same laws. Indeed, one of the earliest indications that this might be so came from a study comparing the energy costs of boring cannon barrels by steam power and by horse power. Whether the fuel was coal or oats, the energy cost of boring a cannon barrel was very similar. This similarity means that thermodynamics is central to our understanding of how organisms work, and even how they are able to exist at all.

Thermodynamics has as its foundation three laws, numbered First, Second, and Third. All concern the behavior of a universe, which is composed of a system and its surroundings. These terms can be rather slippery, and being careless about their meanings can make some very simple ideas seem very difficult. For example, the ther-modynamic universe can encompass something as small as a molecule, a cell, or an organism; especially in biology, we rarely refer to the thermodynamics of the entire universe, as the common meaning of the term implies.

The First Law of Thermodynamics, which constrains the quantity of energy in the universe, is sometimes designated as the law of conservation of energy. It simply states that the total amount of energy in the universe is a constant. It does not limit the energy either to the system or to the surroundings, but the sum of all energy in the universe must be constant. It does not constrain the form the energy can take (that is, it can be potential or kinetic energy, heat energy, electrical energy), nor does it constrain the flow of energy between the system and its surroundings.

The Second Law of Thermodynamics is also known as the law of increasing entropy. This simple law has some marvelously subtle implications for life. The Second Law states that whenever energy does work—whether the system does work on the surroundings, or vice versa— some fraction of the energy is lost to random molecular motion, or entropy (sometimes referred to as "disorder"). Thus, in any universe where there is work being done, there will be a relentless increase in the universe's entropy. It is important to remember that the Second Law does not force increase of entropy on either the system or the surroundings. Likewise, there is nothing in the Second Law that prevents a decrease in entropy (or an increase in order) in some part of the universe. Any decrease in the entropy of one part must be accompanied by a greater increase in the entropy of another part, however. The Second Law demands only that the universe experience a net increase of entropy. Organisms, which are highly ordered systems that can be thought of as transient "pools" of low entropy, exist only by disordering the universe in which they exist.

The Third Law of Thermodynamics is a bit more esoteric, but it is important in that it gives us a thermodynamic definition of temperature. Put simply, it states that there is a lower limit on the temperature of any universe, the point at which random molecular motion, or heat, falls to zero. This is the basis of the absolute, or thermodynamic, scale of temperature, designated by units of kelvins, or K. The zero on the absolute temperature scale is often referred to as absolute zero, because a temperature lower than 0K is impossible (there is no such thing as negative motion). In terms of the more frequently used Celsius or Fahrenheit scales, which are zeroed at "convenient" temperatures, absolute zero is -273.15°C or —459.67°F.

cose and oxygen, it also produces orderliness from disorder.

Orderliness increases by this reaction because a large number of simple molecules is reduced to a smaller number of more complex molecules:

12 molecules (6 containing carbon)

C6H12O6 + 6O2 7 molecules (1 containing carbon)

The increased orderliness means it is easier to keep track of the atoms on the equation's right (orderly) side than on its left (disorderly) side. Look at the carbons. On the left side of the equation, each carbon atom is locked up in one of six carbon dioxide molecules. Nothing about one particular carbon atom tells us anything about what the other five carbon atoms are doing—where they are, how fast each one is going in what direction, and so forth. For a complete description of the system on the equation's left side, we need information about each carbon atom. But when six carbons are brought together into a single glucose molecule, knowledge of one carbon atom tells us a lot about what the other five are doing. Less information is required to describe the equation's right side, and it is therefore more ordered.

Energy comes into the picture as light. "Light" is a bit of a misnomer—it is simply our word for a particular type of electromagnetic energy that comes bundled as particles called photons. Having light come in photons is handy because it allows us to treat energy in chemical reactions as we treat atoms. We know, for example, that the production of one molecule of glucose by a green plant (that is, one with chlorophyll) requires about 48 photons of red light. This lets us rewrite our photosynthetic reaction:

where h is Planck's constant (6.63 X 10-34 J s) and v is the photon's vibration frequency (s-1).2 For red light, for example, the vibration frequency is about 4.3 X 1014 s-1, and the energy carried by a photon of red light is about 2.8 X 10-19 joule. We can now write the photosynthetic reaction with the energy term made explicit:

So, not only can we say that it takes energy to create order in this reaction, we are able to say with some confidence just how much energy is required. And this enables us to illustrate another important feature of how organisms work.

If we turn the photosynthetic reaction around, we get a chemical reaction representing the breakdown of glucose into carbon dioxide and water. This happens when, for example, we burn wood (which is basically glucose), or when we burn sugar as fuel in our bodies in the process of metabolism:

C6H12O6 + 6O2 ^ 6CO2 + 6H2O + energy more orderly ^ disorderly

In this reaction the carbons in glucose are returned to their initially disordered state, and in so doing energy is released. This energy had to come from somewhere. Its source, of course, was the energy initially supplied in the form of photons from the sun. Production of order is a means of storing energy the organism can use later on.

We also know a lot about how photons carry energy. For example, the energy in a photon, expressed in joules (J), can be calculated from Planck's law:

2. If you are not familiar with scientific notation or with the conventions for expressing units of measure, you may wish to take a few moments now and peruse Box 2B.

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