Remember the following sentence, for it is at the heart of this book. Physiology is the science of how living things work.
Physiology these days is a science biased toward the small, concerned with how individual cells and particular molecules in cells work. There is an obvious reason for this—we are the beneficiaries of a scientific revolution, one that heated up through most of the early part of the twentieth century and finally came to a boil in 1953. That was the year James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin figured out (or more properly, told the rest of us about) the structure of DNA. This was a watershed event because embedded within the structure of DNA was the answer to many of biology's fundamental ques-tions—what made us what we were, what controlled our development, what made the complicated chemistry inside cells work right virtually every time? Biologists had been asking these questions for centuries, of course, but understanding the structure of DNA enabled them, for the first time, to ask the questions in a scientifically meaningful way. Since 1953, the progress of molecular biology has been nothing short of triumphant. Like the Copernican revolution in its time, molecular biology is the crowning intellectual achievement of our time.
So, it is easy to forgive my molecularly oriented colleagues when they occasionally slip into arrogance, exemplified by the quotation at the head of this chapter. They've earned it, really. Nevertheless, it is grating: there are, after all, many other interesting ways to think about the world and how it works. For example, consider all the ways one could ponder the workings of automobiles. At the very small scale, we could focus on details of how individual molecules of hydrocarbon fuel are oxidized in the complex environment of the engine's cylinders. Now, this is unquestionably fascinating. It is easy to imagine how it could be crucial to understanding how automobiles work. But small-scale questions such as these are embedded in other important questions, like: how does an automobile interact with a road surface or with the air mass it travels through? Personally, I would be hard pressed to say that the only interesting questions left in automobile design concerned hydrocarbon molecules. I would be similarly hard pressed to agree that the only interesting questions left in biology are to be found at the molecular scale.
So, what are the other interesting questions?
Arthur Koestler has written of the "Janus-faced" nature of life,1 meaning that life always presents two faces to the world. At no matter what scale one looks at it, life may always be described in terms of a nested hierarchy of what biologists call organizational levels. Organisms represent one level of organization, but these comprise numerous organ systems (cardio-respiratory, digestive, nervous), which themselves are made up of organs (say, hearts, lungs, blood vessels, blood plasma, and cells), which in turn are built up from assemblages of tissues (muscle tissue, connective tissue). One can carry this outlook all the way down to the level of individual molecules that, assembled in just the right way, make a living thing. Turning our
1. In Roman mythology, Janus was the god of beginnings and ends. The Romans placed him at the creation of the world and regarded him as the supreme deity, more highly even than Jupiter. His insigne typically showed him with two faces (Janus bifrons), looking in opposite directions, a figure that supposedly symbolizes the confused state of the primordial world as it emerged from chaos. This design was commonly placed over thresholds of gates and doors, symbolizing the transition between the inner and outer spaces of a home or city. In ancient Rome, the cult of Janus was politically powerful, as is reflected in the name of the month (Januarius) that inaugurates the new year.
gaze around, however, we see also that organisms are themselves embedded in larger assemblages. Individual organisms interact with other organisms, whether of the same species or others, and these assemblages of interacting organisms make up the populations, communities, ecosystems, and biomes that collectively constitute the living world.
For the last century or so, biology has mostly relied upon only one of its Janus faces. Biologists today look at life with a determined inward gaze, starting from the organism and working down through the various levels of organization, ultimately ending at molecular biology. Suppose we unmask Janus's other face, however, and take a deliberate outward look? What then do we see? Well, lots of things, really, but one prominent feature is bound to be the macroscopic world in which organisms live—the environment—and how organisms interact with it. One of the premises of this book, in fact, is that it is here, beyond the organism, where the really interesting questions in biology lie.
The science of how organisms interact with their environments goes by various names, including physiological ecology, ecophysiology, and environmental physiology. These outward-looking fields of biology have been largely eclipsed by the triumphant march of molecular biology over the last fifty years, but they have been respectably active. Unfortunately, despite their outward perspective, the work of environmental physiologists and ecophysiologists has been largely one-sided. Speaking of an interaction between an organism and its environment clearly implies two things. One, of course, is the effect of the environment upon the organism, and the second is the effect of the organism upon the environment. Most of modern environmental physiology is focused on the first—that is, the effect ofenvironmental temperature (salinity, wind speed, radiation, pH) on such-and-such an animal, plant, organic process. Rarely are effects acting the other way considered. Nevertheless, the action of the organism on its environment has to be important, and, furthermore, it has to be physiological. In other words, a true interaction of an organism and its environment must result in an extension of physiology outside the conventionally defined boundaries of the organism.
I want to convince you this kind of physiology does exist and that it is, in principle, no different from the physiology that goes on inside an organism. Fortunately, my task will be an easy one, because it relies on a well-established principle about how the insides of organisms work: organisms, like all other things in the universe, are governed in their operations by the laws of that most fundamental of the physical sciences, thermodynamics. Physiology inside an organism, therefore, is fundamentally a problem in thermodynamics. This is an uncontroversial and, as far as I know, a universally accepted assertion about life. To demonstrate that physiology extends outside the organism, all one needs to do is broaden the perspective a bit and show that the principles of thermodynamics don't stop at the organism's skin.
Here we return to the sentence I asked you to remember at the start of this chapter. In thermodynamics, the word work has a very precise definition. We will be getting into the concept of work in more detail momentarily, but suffice it to say that work in the thermo-dynamic sense is only done when energy is made to flow. How energy flows, and how it can be made to do work in the process, is the subject of thermodynamics. Thermodynamics is sometimes presented as a very abstruse and difficult subject, but its basic principles are fairly simple (see Box 2A). These principles govern one of the two fundamental things that organisms do—they channel energy through their bodies and create orderliness in the process. (The other fundamental thing organisms do is to encode and transmit information about how to make copies of them-selves—the molecular stuff.)
Order is something most of us intuitively understand. I am one of those people that are politely referred to as "organizationally challenged"—in the bad old days, I would have been called a slob. My desk is littered with piles of paper, my laboratory has remnants of half-completed projects laying around everywhere. I occasionally try to put my life in order, but I soon stop, because it is just too much work! And this illustrates a very important point about order—it takes work to create it and to maintain it. So while I am lying down until the urge to tidy my office passes, I console myself by reflecting that I am in harmony with one of nature's fundamental laws.
This is more than a glib joke—if left alone, the tendency of the universe seems to be toward increasing disorder, not its opposite. The creation of orderliness runs counter to what, in all our experience, seems to be a fundamental feature of the universe. Nevertheless, order does appear, it appears in many forms, and when it pops up, as it does in living things, it demands explanation. Fortunately, the creation of order by living organisms has been explained, for the most part, and a relationship between energy and orderliness is at the crux.
To illustrate how energy and order are related, let us look at a chemical reaction that is arguably the most important one on Earth: the photosynthetic fixation of carbon dioxide and water into glucose (a sugar) by green plants.
The reaction as it occurs in plants is very complex, but we can state it in a simple form that relates its reac-tants (on the left) to its products (on the right):
light + 6CO2 + 6H2O ^ C6H12O6 + 6O2 disorderly ^ more orderly
Below the chemical formula for the reaction, I have made an assertion: the reaction not only produces glu-
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