Atp

Figure 2.5 The overall scheme of calcite deposition in a calciloblast cell of a hermatypic coral.

CaCO.

CaCO.

used to power the separation of more hydrogen ions across the calciloblast membrane, which keeps calcite deposition going, which keeps photosynthesis going, and so on and so on.

Physiology, the Organism, and the not-Organism

Let us now step back a bit and reflect. I asserted earlier that physiology is essentially how animals use energy to do order-producing work. On this very general level, there are obvious parallels between what goes on in the fish nephron and what goes on in the coral calciloblast. In both cases energy flows through the organism's body (light ultimately, then glucose, and then ATP) to create either order (partitioning ions or other molecules across a membrane in both cases) or potential energy (blood pressure in the fish nephron, con centration gradients of hydrogen ions in the cal-ciloblast). The two examples differ primarily in their frames of reference. In the fish nephron, what is organism and what is not-organism is clear—the environment is outside the organism, and the physiological function takes place inside the animal. In a hermatypic coral, though, the boundary between organism and NOT-organism is not so clear. An important component of the physiological process takes place outside the animal, at the space between the outside surface of the calciloblast and the exposed surface of previously deposited calcite. For the coral, even, two organisms are involved, the boundary between them blurred by their symbiosis. In short, the identity of the organism is very tenuous in hermatypic corals. Yet, the whole system—reef, calcite, coral, and zooxanthellae—func-tions together as a physiological system.

If physiology occurs in both systems, and if the exis tence of physiological function is not dependent upon a clear partition of an organism from its environment, then there seems to be little reason to regard the organism as an entity discrete from its environment. This is the crux of the argument I wished to make in this chapter. If animals use energy to do work on the "external" environment, their activity is as much physiology as when they use energy to do work on the "internal" environment.

We tend to scoff at the beliefs of the ancients. But we can't scoff at them personally, to their faces, and this is what annoys me. —JACK HANDEY, deep thoughts (1996)

chapter three

Living Architecture

Dante set aside a special place in the underworld for people who make the kinds of arguments I made in the last chapter. I suppose it would not be all that bad—I could be consoled by the good company, as Dante sent most of the pre-Christian (pagan) philosophers there as well. But, mea culpa, there I would have been sent, for my crime is sophistry.

These days, sophistry is a Bad Thing: my dictionary defines the word as "deceptively subtle reasoning or arguing," that is, glib argument intended to put something over on us. When a tobacco company lawyer claims, for example, that his company never forced anyone to light up, and so his company should not be held responsible for the social consequences of the public's use of its products, the fact that he is technically correct does not overshadow the rather transparent and cynical agenda he is trying to advance. That is sophistry, as we now know it.

Sophistry has a rather more distinguished ancestry, though. The word is derived from the name given to a group of itinerant pre-Socratic philosophers, the Sophists, or "wise ones." Their principal approach to philosophy was to use logic to explore fundamental questions about man and the cosmos. They seemed not to be popular in their day—Socrates may have been put to death for being too close to them—because they often used their formidable argumentative skills to question the morals and customs of the Athens of their day. They were tolerated by the ruling classes, though, because they were superb teachers of rhetoric, logic, and public speaking, essential skills for successful careers in public life. They were, in other words, a bit like a modern university faculty.

We know most about the Sophists from Plato's Dialogues, and he had little good to say about them. He disliked their fondness for argument for its own sake and what he viewed as their quibbling contentiousness. Most annoying to Plato was their great skill at using logic and rhetoric to advance falsity as easily as truth. To Plato, Sophists were most interested in empty rhetoric, specious logic, and victory in debate.

One of the Sophists' favorite rhetorical tools was a technique known as antilogic, in which a logical proposition is met with a logical counter-proposition, the implication being that both are equally valid. One can then undermine the proposition without having to explore, on its own merits, why the proposition might be correct. What frequently got the Sophists into trouble was their use of antilogic when the matter under scrutiny was a question of morality or social custom. This is a method familiar to us Baby Boomers and now (maddeningly) to our progeny:

• Proposition (father to daughter): No, you are not allowed to have a nose ring.

• Counter-proposition (daughter to father): You wore long hair when you were my age and your parents didn't stop you!

I used antilogic in Chapter 2, in fact. My argument was roughly as follows:

• Proposition: Physiological processes inside organisms are demonstrably governed by thermodynamics.

• Counter-proposition: Certain processes associated with living things, but occurring outside the body, also are demonstrably governed by thermodynamics.

• Ergo: There is no distinction to be made between physiological processes within the body and similar processes without.

All I have proven, of course, is that physiology, as it occurs in "the real world," is governed by the same chemical and physical laws that apply to inanimate physical systems. This is a trivial conclusion, really, but it can very easily be twisted it into absurd conclusions. For example, a thundercloud, as it forms, obeys the laws of thermodynamics as rigidly as a kidney does as it maintains the water balance of a fish. My exercise in antilogic could have as easily led me to assert that there is no fundamental distinction to be made between the origin of thundershowers and the origin of fish urine. And the assertion is as logically correct as it is nonsensical. You can see why the Sophists drove Plato mad.

All I really did in the last chapter was show that there is no logical reason why physiological processes cannot occur outside an organism. I am still far from showing that "external physiology" is a reasonable or even an interesting idea. What I would like to do in this chapter is to show that physiology outside the organism is not only logically possible but also reasonable.

The Inefficiency Barrier?

Conventional physiology encompasses an organism using metabolic energy to do work on its internal environment. A putative external physiology extends the reach of this work to the environment outside the animal. In the last chapter, I asserted that the laws of thermodynamics put no constraints, save one, on the extent of this outward reach. It so happens that this one constraint is a serious one, so it must be dealt with straightaway.

The supposed constraint is imposed by the Second Law, which states that any transformation of energy into useful work must be accompanied by the loss of a portion of that energy as heat:

energy in ^ useful work + heat

The problem is that physiological functions are powered only by energy being made to do useful work— with a few exceptions, heat is pretty much useless to an organism's physiology. And this leads to a problem

Table 3.1 Hypothetical efficiency of filtration work done by the fish kidney.

Process

Process efficiency

Cumulative efficiency

glucose ^ ATP 38%

ATP ^ increase in blood pressure 21%

blood pressure ^ filtrate production 70%

38% of 36% = 13.7% 21% of 13.7% = 2.87% 70% of 2.87% = 2.01%

we might call the inefficiency barrier. The inefficiency barrier, if it exists, may force us to the conclusion that, although external physiology might be possible, we must conclude that it is energetically unreasonable.

Organisms, like all machines, are inherently inefficient. Furthermore, the inefficiency is cumulative, which means that the more useful things an organism tries to do with a parcel of energy, the less efficiently it will do them. This is bad news: most organisms do work in sequences of small steps between the initial input of energy to a physiological "engine" and the work ultimately done by it. If each step wastes some energy, the accumulating inefficiency may mean that little energy will be left over from internal physiology to extend its reach outward. This "inefficiency barrier" may explain why organisms have tangible boundaries even though there is no thermodynamic reason they should.

To illustrate, consider how energy was made to do the work of filtration in the fish kidney, as described in Chapter 2 (Table 3.1). The process begins with a parcel of energy initially stored in sugar and continues as that energy is made to do work in several stages before it dissipates ultimately to heat. Each step has a characteristic efficiency, always less than 100 percent, and the degradation of useful work to heat is cumulative at each step. At the end, once the filtrate is made, very little energy is left, only about 2 percent. This pyramid of inefficiency piled upon inefficiency offers a rather bleak prospect for organisms being able to power physiology outside their bodies.

I can think of two ways to confront this problem.

The first is a bit of sophistry that may earn me additional time on my infernal sentence. I will employ it anyway, because this time it is the proposition, not the counter-proposition, that is probably in error. The second is the argument that even if an inefficiency barrier precludes an organism powering its own external physiology, there are ways of circumventing this limitation.

Negation by Minimization

Another of the sophist's useful tools is negation by minimization. Equating "small" with "insignificant" is a convenient way of avoiding serious thought about something, or (even better) of diverting someone else from thinking seriously about it. We all are depress-ingly familiar with this technique. We see it used all the time in what we call, without irony or embarrassment, the political "debate" of our fractious age. Some examples: "The tax money spent on (my) [program X] is so small that cutting it will have no significant effect on the national debt. It would be far better to cut (someone else's) [program Y]." Or: "The taxes paid by taxpayers (like me and my friends) earning over $3 million a year are so small a portion of all taxes levied that raising them will have no measurable impact on overall tax revenues. Better to raise taxes on (those other) taxpayers (over there) that earn less than $50 thousand a year." Positing an inefficiency barrier as an obstacle to external physiology uses the technique of negation by minimization. If only a tiny fraction of the energy passing through an animal is left to do useful work, then anything else an organism could do with it, like powering external physiology, is not worth noticing.

In politics, this kind of argument is smoke and mirrors. So it is in biology. The obvious antidote to negation by minimization is to show that what we are being asked to ignore is not so negligible after all. So, let's take a closer look at the propositions that underlie the inefficiency barrier. Consider that chain of inefficiencies I just outlined. Are organisms really that inefficient?

Well, yes, and no. Consider the claim that the conversion of glucose to ATP is about 38 percent efficient. Where does this number come from? It is a fairly straightforward calculation based upon a quantity known as the standard free energy (See Box 2C), abbreviated AG' (pronounced "delta gee nought prime"). The standard free energy quantifies the energy in a chemical reaction that is available to do work. For the oxidation of glucose to carbon dioxide and water, for example, the AG ' is 2.82 MJ mol-1. As energy is transferred from glucose to ATP, its release from glucose must be coupled to the addition of a phosphate to ADP. This reaction has a AG' of about -30.5 kJ mol-1, that is, roughly 30.5 kJ of energy is required to phosphorylate a mole of ADP. If a cell completely oxidizes a mole of glucose, it will produce thirty-six moles of ATP. Phosphorylating 36 moles of ATP therefore requires 30.5 kJ mol-1 X 36 mol = 1.09 MJ of energy. The efficiency of conversion of glucose to ATP, therefore, is simply 100 X (1.09 MJ/2.82 MJ) = 38 percent.

Standard free energies are rigorously measured quantities, so it would seem our calculation of inefficiencies is built upon a rock-solid foundation. It is not as solid as it might appear, though. The problem with standard free energies is not that they are inaccurate, it is that they are incomplete. A standard free energy of a reaction is so called because it is quantified under certain standard conditions. Measurements are made under standard conditions so that different reactions may be compared and predictions may be made about whether a particular reaction will go. As useful as this practice is to biochemists, it unfortunately says little about the actual energy yields of particular reactions when they occur inside cells. Indeed, by manipulating the conditions in which reactions take place, it is possible to make significant improvements in yields of useful work, even from reactions that supposedly are very wasteful of energy. For example, the synthesis of a single peptide bond operates at a standard efficiency of about 23 percent.1 Changes in concentrations of products and reactants, local pH, and temperature that may occur in an organism's cell may raise efficiencies to a theoretical maximum of around 92-96 percent (depending upon how the costs of infrastructure, like DNA and mitochondria, are factored in).

Reactions in most cells do not approach this theoretical limit, but they nevertheless do much better than their standard efficiencies would suggest. Bacteria, for example, operate at total conversion efficiencies2 of around 60 percent. Protozoan cells do a little poorer on average, but still pretty well, about 50 percent. Most animal cells are similar, operating at 50-60 percent efficiencies. Some predatory protozoa seem to push the envelope, with efficiencies of around 85 percent. So, the Second Law seems a bit less formidable a barrier to an external physiology, at least when we take into account the conditions that may occur in a living cell.

This is all very nice theoretically, but does it mean that organisms will have energy to spare once they have taken care of their physiological housekeeping? On the face of it we might say no: would it not make more sense for an organism to keep the energy inside

1. A peptide bond links two amino acids into a dipeptide. A protein, consisting of a linear chain of n amino acids, contains within it n - 1 peptide bonds. Synthesis of a peptide bond requires about 20.9 kJ mol-1. Two ATPs and 1 GTP are required, providing energy of 3 X 30.5 kJ mol-1 = 91.5 kJ mol-1. The efficiency is therefore 100 X (20.9/91.5) = 22.8 percent.

2. The total conversion efficiency is the efficiency of converting energy in food to new organism. These figures therefore reflect the net efficiencies of a sequence of steps, including at minimum the conversion of food energy to ATP and the use of ATP to power the synthetic reactions required for growth.

to be used for other useful chores, like growth or reproduction? It turns out there is a convenient way to examine this matter independently, by looking at how efficient the energy transfers are between organisms. Such transfers of energy usually involve one organism being eaten by another, what biologists delicately call trophic interaction.

The efficiency of a trophic interaction can be roughly gauged by measuring how much of one organism (the eaten) is needed to support another (the eater). For example, green plants convert light energy to glucose, use it to power growth, and in the process accumulate stored energy, the so-called biomass. Herbivores (predators of plants, really, although it hardly seems sporting to call them that) take this energy for their own support. If the use of energy by an herbivore is completely efficient, a certain biomass of plant tissue should be able to support the same biomass of herbivore. If, on the other hand, the herbivore is only, say, 10 percent efficient, a certain plant biomass will only an herbivore biomass one-tenth its size.

If we take this argument to its logical conclusion, we could arrange all organisms in nature in a kind of trophic pyramid (Fig 3.1), where (assuming an arbitrary efficiency figure of 10 percent) the total biomass of herbivores is 10 percent of the plant biomass, and the biomass of primary predators (things that eat herbivores) should be 10 percent of herbivore biomass (or 1 percent of plant biomass), and so forth. The pyramidal shape of these trophic interactions is enforced by the supposedly inevitable wastage of energy as heat as it passes through living organisms. The greater the internal wastage, the less energy will be available for powering the physiology of other organisms, and the more sharply constrained the trophic pyramid should be.

What, then, do trophic interactions tell us about just how efficient organisms are? This issue has long been a staple of ecological research, and it turns out there is quite a lot to tell—so much, in fact, that there is no way to summarize it without also trivializing the topic. Nevertheless, one can say, as a crude generalization, a.

Primary producers

Primary consumers

Secondary consumers

Tertiary consumers

+100

+1000

Figure 3.1 The supposedly inefficient flows of energy through ecosystems. a: The passage of energy from one type of an organism to another inevitably involves the loss of some energy as heat. The efficiency is arbitrarily set to 10 percent in this example. b: A simple pyramidal food web in an intertidal community on the Washington coast. c: A food web from an intertidal community in the Gulf of California. [b and c after Pimm (1982)]

Figure 3.1 The supposedly inefficient flows of energy through ecosystems. a: The passage of energy from one type of an organism to another inevitably involves the loss of some energy as heat. The efficiency is arbitrarily set to 10 percent in this example. b: A simple pyramidal food web in an intertidal community on the Washington coast. c: A food web from an intertidal community in the Gulf of California. [b and c after Pimm (1982)]

that there is both good news and bad news for the idea that organisms are inherently inefficient. For example, some types of organisms, like mammals and birds, which divert considerable quantities of energy to the deliberate production of heat, are decidedly inefficient, operating at "ecological efficiencies" of 2-3 percent. Other types of vertebrates seem to operate at efficiencies around 10 percent. Most invertebrates, excluding the insects, do better, with efficiencies ranging from about 20 percent to 35 percent. Insects operate at the greatest ecological efficiencies of them all: with 39 percent efficiencies for herbivore species and up to a whopping 56 percent for insect carnivores (another reason why insects will ultimately conquer the world).

In general, highly efficient organisms not only power more work inside their own bodies, but they also produce more work energetically "downstream." Ecologically, this relation is sometimes reflected in the length of a chain of trophic interactions, essentially how many times bigger fish can eat littler fish. Ecosystems with poor productivity, for example, support simple trophic pyramids that can power only a few trophic exchanges before the energy is all dissipated as heat (Fig. 3.1). Highly productive ecosystems, on the other hand, "frontload" sufficient energy into the system, powering parcels of energy through many more trophic levels, as many as 10-15 or so. A productive ecosystem is better described as a complex food web than as a pyramid.

Clearly, inefficiencies exist in the transfers of energy both through and between living things, and these inefficiencies occur at every level—cellular, organismal, and ecological. Also, the inefficiency is cumulative and multiplicative. As a consequence, there is a limit on the extent that physiological work can reach beyond the organism's conventionally defined outer boundary. Despite this limitation, there seems to be little reason to minimize the possibility of an external physiology to the point of negligibility. Efficiencies of physiological work are subject to improvement through engineering, and there is every reason to expect—both a priori on the basis of natural selection, and a posteriori on the basis of empirical observation—that species will generally improve the efficiency of their internal physiology. Certainly, energy flowing through organisms demonstrably supports the physiology of other organisms energetically downstream from them. Why, then, should we suppose this energy could not be used by an organism to support its own physiology outside (and also energetically downstream from) itself?

Circumventing the Inefficiency Barrier

Another interesting way an organism can work around its internal inefficiency is to use energy that does not pass through its own chemically based physiology. By this means an organism can perform some interesting energetic jujitsu.

Organisms channel energy through themselves, using it to do work in the process. Consequently, organisms are positioned in the middle of a stream of flowing energy, much in the manner of a hydroelectric plant in a river. Conventional (internal) physiology relies on a particular form of energy flow—specifically, that from chemical potential energy stored in the orderliness of complex chemicals like glucose and ATP. Aside from the initial capture of light energy by plants, nearly all the energy flowing through organisms is in the form of various chemical transformations: glucose to ATP, and ATP to various tasks, like synthesis, transport, and mechanical work. Ultimately, it is all dissipated as heat. This energy stream, because it primarily involves chemical transformations under the direct control of organisms, we might call the metabolic energy stream (Fig. 3.2). The inefficiency barrier, whether it is big or little, is a problem peculiar to the metabolic energy stream.

Energy flows through the environment in other forms, however. The potential rate of energy transfer from the Sun to the Earth is prodigious—about 600 W m-2, averaged through the year.3 Of this, only a relatively small fraction, on the order of 1-2 percent, is

3. To put this number in perspective, compare it with the energy consumption rate of a small four-person house located in the northeastern United States (mine). My family's average yearly energy consumption is about 19,500 MJ for electricity, and about 84,300 MJ for natural gas, a total of about 104,000 MJ. This worked out to a total energy consumption rate of about 3.3 kW. Our house occupies a small plot about 350 m2. Thus, our typical house consumes energy at a rate of about 9 W m-2,

Physical energy stream

Physical energy stream

Metabolic energy stream

Figure 3.2 The multiple energy streams in the environment.

climate physiological PE gradients physiological PE gradients progeny captured by green plants. The rest, if it is not reflected back into space, is available to do other things. The excess can be considerable: although some natural surfaces reflect as much as 95 percent of the incoming solar beam, many natural surfaces reflect much less (Table 3.2), on average about 15-20 percent. The remaining absorbed energy is then capable of doing work, like heating up surfaces, moving water and air masses around to drive weather and climate, evaporating water, and so forth. Eventually, it all dissipates ultimately as heat, but it does work along the way, forming a physical energy stream that runs roughly parallel to the metabolic energy stream (Fig. 3.2).

If organisms could tap into this parallel stream of physical energy, they could power external physiology while circumventing the efficiency limits of the metabolic energy stream. Capturing even a small part of the Sun's energy to do physiological work would confer enormous benefits to an organism. For example, temperature regulation is one of the major metabolic costs for warm-blooded animals like birds and mammals.

roughly 1.5 percent of the solar energy delivered. This abundance of energy reaching the Earth's surface is one of the things that keeps advocates of solar energy hopeful.

Maintaining a steady body temperature means producing heat as rapidly as it is lost to the environment, and the colder it gets, the greater this cost will be. The energy for production of heat is supplied by ATP, that is, it comes right out of the metabolic energy stream. This is a waste—it is akin to heating your house by burning dollar bills. Roadrunners (Geococcyx californianus) are an interesting counter-example to this rule. On cold mornings, when metabolic costs for heat production would ordinarily be high, road-runners will sometimes be seen sunning themselves. Obviously, heat the roadrunner absorbs from the sun is heat that will not have to be generated from ATP, and savings in metabolic energy will result. Roadrun-ners' bodies are even modified to increase the benefit they derive from sunbathing. Their back skin has several melanized patches that can be exposed directly to the sun when the back feathers are lifted up, like slats in a venetian blind. The additional heat from the sun can lower a roadrunner's metabolic costs for thermoregulation by about 40 percent. It takes little imagination to see that animals might build structures external to their bodies to take advantage of such benefits, and we will be exploring many of them in the later chapters of this book.

Table 3.2 Surface reflectivities of some representative natural surfaces (after Rosenberg 1974).

Surface

Short wave reflectivity (%)

Fresh snow

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