## Chlamydomonas Nivalis Pattern

Figure 3.5 a: Structures can be made to do adaptive work by rectifying the flow of energy across the structure, symbolized by the diode. b: More sophisticated adaptive work can result if the structure can adaptively control energy flow as if through a transistor. c: Structures can store environmental energy in a capacitor, Cstr.

Figure 3.5 a: Structures can be made to do adaptive work by rectifying the flow of energy across the structure, symbolized by the diode. b: More sophisticated adaptive work can result if the structure can adaptively control energy flow as if through a transistor. c: Structures can store environmental energy in a capacitor, Cstr.

PEen Re

PEen Re

larger current across two other terminals, the collector and the emitter. Current flows only one way in a transistor—from the collector to the emitter. Consequently, a transistor can be thought of as a switchable diode controlled by the current at the gate. Adaptively regulate the gate current and you adaptively regulate the flow of current through the transistor.

Consequently, we could analogize energy flow through the structure as a transistor, Qstr, controlled by some kind of input, whether from the animal that built the structure or from the environment acting on the structure (Fig. 3.5b). At its simplest, energy flow through the structure would be represented by another conditional equation:

J = (PEstr - PEEarth)/Rstr [IF (Condition = TRUE)] [3.9]

Finally, in all these equivalent circuits, circumstances may arise in which the energy that comes into the structure, from whatever source, is stored for use at a later time. If the energy coming in acts to heat the structure, for example, the increased temperature is actually stored energy that can be used to do other work when the PE gradients between the structure and environment are unfavorable. Suppose, for exam ple, a structure uses heat energy to evaporate water and move it around in some way for the animal's benefit. If the structure can store this energy (as a high temperature) during the daylight hours, when the sun is contributing lots of light energy, it could be used to power work after the sun goes down.

Again, storing electrical energy is something that electrical engineers must do all the time. They store electrical energy in capacitors, which can be placed into an electrical circuit in a variety of configurations, depending upon what might need to be done. For our purposes, we will illustrate in an equivalent circuit the simplest configuration: a resistor in parallel with a capacitor, Cstr (Fig. 3.5c). Here, when the PE gradient is favorable for the environment doing work on the structure, energy flows in. Some of the energy does work by flowing down Rstr, and some is stored in the capacitor. When the PE gradient for the environment doing work on the structure is not favorable, the stored energy will then flow out of the capacitor and down Rstr, continuing to do work.

By now, I hope that the path to a notion of physiology outside the organism, if not clear, is at least reasonably visible. In the next chapter we turn to "real biology," which will occupy the rest of this book.

The penalty for laughing in the courtroom is six months in jail: if it were not for this penalty, the jury would never hear the evidence. —h. l. mencken chapter four

### Broth and Taxis

In Chapter 2, I committed the sin of sophistry, and I have just spent Chapter 3 trying to exculpate myself. I confess that I committed another intellectual sin in the effort—I relied on an "occult force" as an explanatory tool.

Now, this sounds very bad: these days, the word occult evokes images of magic tricks taken rather too seriously, or things even more sinister. But being occult is not so bad, really. The literal meaning of the word is simply "unseen" or "obscured from view." An occult force, therefore, is simply an unseen cause of something. In fact, occult forces are used by scientists all the time because they are actually very handy. For example, gravity is an occult force, in that it is unseen and it remains mysterious in certain ways. Yet we know pretty well how it behaves, well enough to calculate positions of planets or predict trajectories of spacecraft with uncanny accuracy. And no one (at least no one I know) is prepared to dismiss gravity as some crackpot idea or to lump Isaac Newton together with occultists like Uri Geller.

I invoked an occult force as a means of linking together orderliness and energy. Although I tried to show with my fairly simple examples that energy and order are related, I implied a causal relationship between the two that, like the workings of gravity, seems almost magical:

Whenever energy flows through a living thing, order is created.

One cannot contemplate questions biological for very long without being struck by this apparently mystical ability to create order.

In this chapter, I'd like to lift the veil a bit on the re-

lationship between energy and orderliness. I will do so by exploring, in some detail, a remarkable example of how order arises from disorder, seemingly spontaneously, simply because energy flows through a living entity. This is something organisms commonly, albeit wondrously, do, as when a cell assembles a complex and highly ordered protein from simpler amino acids. In the example to follow, though, the order appears not within the organisms themselves but in their environment, on a scale many orders of magnitude larger than the organisms producing it. In describing this large-scale orderliness, I hope also to enlarge on my claim that structural modification of the environment can power a physiological function outside the body.

Large-Scale Orderliness in Cultures of Microorganisms

First let me describe the phenomenon I have in mind. The organism is a swimming protozoan, something like the flagellated Chlamydomonas (Fig. 4.1). These organisms are fairly easy to culture in the laboratory, and if you have studied zoology or protozoology you probably have heard of them. The phenomenon occurs in a liquid culture of Chlamydomonas, at a density of about a million cells per cubic centimeter. You'll want to view the petri dish with transillumination, that is, lit from below so light passes through it to your eyes above it.

If you stir the culture up, it will appear uniformly cloudy, or turbid, like a jar of apple juice that has begun to ferment in the refrigerator. The reason the culture looks cloudy is that the billions of cells floating in the culture each reflect and refract light passing through them. When the cells are randomly distributed through the culture, the scattering of light beams through it is also random, and turbidity is the result.

Turbidity of the culture, arising as it does from a random, disordered distribution of the protozoans, should be favored by the Second Law. While we might have helped the culture become random by mixing it up, once it was randomized, the Second Law says it should have stayed that way. But in fact, the culture will stay

Figure 4.1 The single-celled microorganism Chlamydomonas. [From Kudo (1966)]

turbid only for as long as we keep mixing it. If we stop mixing, its appearance begins to change dramatically. Within a few minutes, the uniformly cloudy suspension begins to develop spots, focal areas of darkness, easily distinguishable visually from brighter areas surrounding them (Fig. 4.2). Eventually, the dark spots begin to merge and ramify, forming beautiful patterns of interleaved light and dark bands. The dark areas, of course, are regions where microorganisms have concentrated at densities high enough not just to scatter light coming through the culture but to block it. The light areas, just as clearly, are regions where organismal densities are low and light beams have a "straight shot" through the culture to our eyes.

The Difference between Thunderstorms and Organisms So, what's going on? A closer look at the structure of these spots or bands gives us clues about how they are generated. When viewed from the side, the dark spots are revealed to be plumes, flowing columns of curtains plumes

Figure 4.2 The development of bioconvection patterns in a suspension of Chlamydomonas nivalis. The photograph looks down on suspensions of Chlamydomonas in two flat culture dishes. Culture in the top dish is 7 mm deep, 4 mm deep in the bottom dish. The inset shows plumes, indicated by black spots, and curtains, indicated by thick black lines. [After Kessler (1985a)]

Figure 4.2 The development of bioconvection patterns in a suspension of Chlamydomonas nivalis. The photograph looks down on suspensions of Chlamydomonas in two flat culture dishes. Culture in the top dish is 7 mm deep, 4 mm deep in the bottom dish. The inset shows plumes, indicated by black spots, and curtains, indicated by thick black lines. [After Kessler (1985a)]

microorganisms, like the downdrafts from a storm cloud. Similarly, the dark bands are downward-flowing aggregations, but in this case they are organized into curtains, walls of downward flow rather than columns (Fig. 4.3). Once these concentrated plumes or curtains reach the bottom of the container, the microorganisms are dispersed sideways. Eventually, they make their way back to the top, sometimes by concentrating in upwardly driven plumes, sometimes simply by swimming to the top in the spaces between the plumes, eventually to be collected again into a downward-moving plume. The net effect is a circulatory movement of microorganisms between the upper surface of the culture liquid and the bottom of the culture dish (Fig. 4.4).

1. Convection literally means "to carry" and it refers to the transport of something (for example, heat, mass, or momentum) by fluid flow.

This kind of flow pattern, familiar to anyone who has baseboard heaters in the home, is known technically as a convection cell.1 When a baseboard heater warms cool air near the floor, the air becomes less dense, and therefore buoyant, so it rises to the ceiling. As the heated air rises, it loses its heat to the cooler walls, windows, and surrounding air until it again becomes dense enough to sink to the floor and eventually make its way back to the heater. The result is a doughnut-shaped pattern of air flow in the room, where air rises at the walls, collects at the ceiling, and is forced down in a concentrated plume at the center. A similar process, albeit in a much more com-

Begin t = 20 s t = 40 s t = 80s t = 120 s t = 180 s t = 200 s

Figure 4.3 The development of bioconvection plumes as viewed from the side, in a culture of the planktonic protozoan Stenosemella nucula. [After Kils (1993)]

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