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3. Those who are well versed in chemical thermodynamics will recognize that I am painting a sort of "worst-case" scenario here. The figure of 36 percent efficiency is based upon a particular set of conditions and concentrations of reactants and products known as the standard condition; specifically, all products and reactants are at a concentration of 1 mole per liter, a temperature of 25oC, and a pressure of 1 atmosphere. Cells, of course, are rarely at the standard conditions, and this allows them to be more efficient than the 36 percent figure just quoted.

The energy from the 48 photons that was not captured as order in glucose was therefore lost as heat, warming

Indeed, some chemical reactions in cells approach energy efficiencies of 95 percent or more. But the point made below remains the same—the reactions will never be perfectly efficient— and some of the energy in the transformation will always be lost as heat.

whatever container (universe) the reaction took place in. The role of this heat is interesting, because increasing the temperature of a substance generally disorders it. Indeed, we can state the photosynthetic equation in a more general form that describes any work-producing transformation of energy:

energy in = work energy + heat energy Or, we can be more radical and state that:

energy in = production of order + production of disorder

Thus, whenever energy is made to do work, some portion of it ultimately ends up increasing the disorderli-ness of the universe. This is the Second Law of Thermodynamics (henceforth simply the "Second Law").

The ATP Cycle

When glucose is broken down to carbon dioxide and water, the energy stored in the glucose molecule is ultimately lost as heat. When glucose is burned directly, all the stored energy is converted to heat in one step. Organisms, however, take the energy stored in glucose and use only a portion of it to do chemical work;the left over energy is stored in another form, to be used later as needed. The ability to store chemical work is central to an organism's ability to use energy.

Metabolism is a process of controlled combustion of fuel, usually glucose. The energy released from glucose can be made to do physiological work only if it is coupled somehow to an energy-demanding process. In nearly all organisms, coupling is done through an intermediary chemical that carries the energy from glucose to the chemical reaction that needs the energy. This intermediary is a nucleotide, adenosine, that carries energy in bonds that link phosphate molecules to it. Most commonly, the adenosine takes part in a cyclical reaction between its diphosphate form, adenosine diphosphate (ADP), and its triphosphate form, aden-osine triphosphate (ATP).

When the energy in glucose is released by metabolism, some of it is used to add a phosphate ion, usually written Pi, to ADP, to form ATP:

The energy comes from coupling this reaction, the phosphorylation of ADP, to the release of energy from glucose:

glucose + oxygen — carbon dioxide + water + energy energy + ADP + P, - ATP + heat

ATP will release energy if it subsequently loses its third phosphate group:

Any other energy-requiring, or order-producing, reaction can be driven, then, simply by coupling it to the release of energy from ATP:

The work thus done could be used to create other kinds of energy, like order, mechanical work, electrical potential energy, and so forth.

To recap, thermodynamics says four important things about how animals work. These are:

1. Animals use energy to produce order.

2. Order can be used as a store of energy that can be tapped at a later time to do work.

3. The amount of order produced is limited by the quantity of energy available to do work.

4. The amount of order produced is limited further by the inescapable inefficiency of any order-producing process.

Thermodynamics and Physiology: Two Examples

I would now like to describe two physiological processes and how energy drives them to create order. The first, formation of urine by the fish kidney, is an example of "blood-and-guts" physiology taking place within a well-understood organ inside a tangible organismal boundary. The second is concerned with the deposition of calcium carbonate by a reef-forming coral. This process is also reasonably well understood, but the boundaries between what physiological activity is inside and what is outside the organism are somewhat blurred. The obvious conclusion to which I want to lead you is that the common features of these two processes do not include a boundary between an organism and its environment.

Water Balance in a Freshwater Fish

The body water of fishes, like that of most vertebrates, is a weak solution of salts and other small solutes: roughly 0.9 percent of the mass of blood plasma, for example, is sodium chloride, common table salt in solution. Fresh water, of course, has far fewer solutes in solution. A fish in fresh water can be thought of as two bodies of water, each differing in composition and separated from one another by the fish's skin. This is order: the orderliness is manifest in the separation of solutes and water into two compartments, the fish and the environment. Because the universe does not like order, the Second Law will dictate that this ordered system be driven to disorder. Disorder in this case will arise as solutes diffuse from the fish to the surrounding water, and as water flows into the fish by osmosis.4

4. Osmosis and diffusion both refer to the movement of matter under the influence of a difference in potential energy. They differ in some crucial respects, though. In an aquatic medium, diffusion refers to the movement of solute from a region of high solute concentration to a region of low solute concentration— down the concentration gradient, it is said. Osmosis refers specifically to the movement of water from a region of low solute concentration to a region of high solute concentration. Osmosis is sometimes referred to inaccurately as water diffusion, because a solution with high solute concentration is also a solution with diffusional efflux of a. solutes influx of water influx of water water

Figure 2.1 The water and solute balance of a freshwater fish. a: The fluxes of water and solutes are driven by gradients of potential energy between the fish and its environment. b: The fish's physiological response is to drive fluxes of water and solutes in opposite directions.

water

Figure 2.1 The water and solute balance of a freshwater fish. a: The fluxes of water and solutes are driven by gradients of potential energy between the fish and its environment. b: The fish's physiological response is to drive fluxes of water and solutes in opposite directions.

Both processes act to equalize the concentrations of solutes on either side of the fish's skin (Fig. 2.1). Because the volume of water in which a fish lives is many times greater than the volume of water contained in its own body, the major change of concentration will occur inside the fish.

The dilution of its internal fluids constitutes a mortal threat to the fish, and the fish's defense is to use energy from glucose to reimpose order(a more concentrated solution). First, by filtering blood through the kidneys, the fish produces large quantities of very dilute urine. This pumps water out of the fish as fast as it comes in by osmosis, keeping the total water content of the fish roughly constant. Second, the fish keeps solutes within its body by using energy to take solutes low water concentration, and vice versa. Water thus moves down its concentration gradient, but it moves against the solute concentration gradient. The analogy is useful, but inaccurate. I will return to the principles of osmosis and diffusion in more detail in later chapters.

from the dilute surroundings and pump them into its body as fast as they are lost by diffusion. Again, this keeps the solute concentrations in the fish's blood high and relatively steady.

Let us now review the physiology of this process. Fishes use two organs, the kidneys and the gills, to keep internal solute and water concentrations steady in the face of thermodynamic assault. The kidneys are primarily responsible for handling the water and salts, and the gills are primarily responsible for handling salts. Let us turn first to the kidneys.

The fish kidney comprises numerous subunits known as nephrons. The general form of the nephron is a tubule, which is closed at one end and opens at the other to the outside of the body through a pore or duct.5 The nephron is surrounded by a network of blood vessels that deliver blood to the tubule and return water and solutes to the blood. It is the job of the nephron to produce urine from blood. Production of urine begins at the closed end of the nephron, and as it is produced, it is pushed out the open end of the nephron to leave the body.

Nephrons work largely by two processes, filtration and reabsorption (Fig. 2.2). Filtration occurs at a junction between the blood and nephron involving two structures: a knot of capillaries, called the glomerulus, and a cup-shaped expansion of the end of the nephron tubule, the Bowman's capsule. The glomerulus sits enveloped in the hollow of Bowman's capsule, and together they form a porous filter between the blood and the tubule. As blood is pumped into the nephron, water, salts, and other small solutes are forced across the glomerulus into the tubule. The resulting liquid is called filtrate: its composition is essentially blood plasma minus blood cells and large proteins left behind in the blood.

Filtration has two interesting features that both help and hurt our freshwater fish in its quest to maintain its

5. Normally, the many ducts emerging from the nephrons merge into a single large tube, the ureter, that drains the assemblage of nephrons that constitute the kidney.

blood blood

(low pressure)

Figure 2.2 The formation of urine in the vertebrate nephron. Blood at high pressure is filtered at the junction between the capillary and the tubule. In the tubule, salts are reabsorbed back into the body, leaving only water to be collected by the duct.

(low pressure)

Figure 2.2 The formation of urine in the vertebrate nephron. Blood at high pressure is filtered at the junction between the capillary and the tubule. In the tubule, salts are reabsorbed back into the body, leaving only water to be collected by the duct.

internal environment. First, filtration produces a lot of filtrate. This helps the fish because it gets the excess water out of the blood. A problem arises, though, because the filtrate's concentration of small solutes, like salts, is virtually identical to the blood's. Consequently, one of the costs of producing lots of filtrate is a loss of solutes. This is undesirable, because one of the fish's problems, after all, is loss of solutes to the environment by diffusion. It does the fish no good if the problem is compounded by further loss of salts via the urine. Luckily, a solution to the problem is available: reabsorption. As the filtrate passes down the tubule, the solutes are transported out of the filtrate (reabsorbed), back into the blood. As solutes are removed from the urine, their concentrations in the filtrate decline. The urine that is produced, therefore, is volumi-nous—because of the high rate of filtration—and very dilute—because of the high rate of reabsorption. The urine that is excreted from the fish is essentially water, whose return to the environment offsets the osmotic flow of water into the organism.

The recovery of solutes from filtrate is only part of the fish's process of maintaining high-solute concentrations in the blood. At the fish's gills are specialized cells, known as chloride cells, that use ATP energy to transport chloride ions from the water into the blood; these ions then pull positively charged sodium ions along with them. Because salt is being transported from the dilute source of fresh water into the relatively salt-rich blood, energy is required to pump the chloride. The inflow of salts across the gills effectively offsets the diffusional loss of salts across the skin.

In the midst of all this movement of water and salts, transformations of energy and the creation of order are taking place. Filtration requires energy to make it go because filtration is an order-producing process. One form of orderliness is the separation of molecules across a membrane. In the nephron, proteins and other large solutes end up being concentrated in the blood, and water and small solutes end up in the filtrate. The energy to power this separation comes from ATP fueling the fish's heart muscle. The work done is transformed by the heart into an elevated blood pressure, another form of potential energy. This potential energy drives and separates the large and small solutes across the glomerulus (Fig. 2.3).

ATP is also used to drive reabsorption. This process, too, is productive of order and therefore requires energy to make it go. Reabsorption produces order because solutes are moved from dilute filtrate into more concentrated blood, against their natural tendency to intermix and reach an equilibrium concentration. The separation is accomplished by protein molecules, known as active transport proteins, embedded in the inner membranes of the nephron's tubules (that is, facing the filtrate). These bind solutes and then release energy from ATP. The energy so released forces the transport molecule to change shape, with the consequence that the bound solute is moved across the tubule wall, from the filtrate into the blood. The transport cells in the gills work in very much the same way. In both cases, energy is used to create order—by separating solutes across a membrane.

There is nothing in these physiological processes that is not well understood energetically. The fish must maintain order (a high solute concentration) in the face of an environment that promotes disorder (water with a low solute concentration). Maintaining order requires work to be done. The energy to power this work comes from potential energy stored in glucose, and the energy released is transformed in several ways as it is made to do work: first into chemical potential energy in ATP, then into mechanical deformations of heart muscle proteins, then into pressure within the fish's capillaries, a form of potential energy that is used to separate proteins from smaller solutes. ATP also promotes the mechanical deformation of transport proteins, which are enabled then to pick up solutes and move them across membranes. All this is conventional physiology, and the operation of simple thermo-dynamic principles is clear.

Calcium Carbonate Deposition in Hermatypic Corals Now let us turn to another interesting physiological process: the deposition of calcium carbonate by polyps of reef-forming (hermatypic) corals.

Corals are coelenterates, related to the jellyfishes and sea anemones. They come in a variety of forms, all very simple in organization, and they are often colonial, with large numbers of individuals cooperatively occupying a space together. One form this coopera-

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