Blood (high pressure)

filtration filtration

Blood (low pressure)

Figure 2.3 The transformations of energy in the production of filtrate. Energy in ATP is converted by the heart to blood pressure. The difference in pressure between blood vessels and the tubules of the nephron does the work of filtering the fluids of the body.

tion takes is the deposition of minerals to build structures, coral reefs being the most spectacular. I will have much more to say about corals in Chapter 5.

A coral reef consists of two parts, one living and the other not. The living part of a coral reef is a thin layer of coral polyps growing on a foundation of calcium carbonate (CaCO3), or calcite, secreted and laid down by cells at the base of the polyp, which are known as the calciloblastic ectoderm—the calciloblast, for short. As the generations of polyps come and go, the reef's mineralized support therefore grows. The end result of this process, multiplied across the millions of individual polyps, is the continuous accretion of layers of cal-cite into a massive reef.

Crucial to the ability of corals to build reefs is an interesting house guest residing in the tissues of the polyps—tiny protozoa, known generically as dinoflagellates ("terrible whip bearers," for the pair of large flagella they sport). Dinoflagellates are common members of the plankton that float about in the surface waters of oceans, lakes, and streams. When dinoflagellates occur as corals' house guests, or symbi-onts, they are known as zooxanthellae ("little yellowish animals"), because they contain a variety of interesting pigments, some of which impart to corals their spectacular colors.6 The pigments also help the zooxanthellae photosynthesize, just as the green pigment chlorophyll aids green plants. There is thus a nice mutual arrangement, or symbiosis, between the coral polyps and their resident zooxanthellae. The zooxanthellae get a nice sheltered home inside the polyps' bodies, and the corals have little "food factories" living inside them, producing glucose from carbon dioxide and water and powered by sunlight.

6. Coral bleaching has recently become a concern for scientists, as there is an apparent epidemic of it among Caribbean corals. The bleaching results from the expulsion of the zooxanthellae from the corals' cells, which is thought to be a stress response. The potential agents for stress include disease, pollution, and high temperature. Warming of the seas may be the culprit in the current epidemic of coral bleaching, and it is thought by some to be a harbinger of global warming.

All corals are capable of producing calcite, whether or not they contain zooxanthellae. However, there is a strong correlation between the ability to build reefs— that is, to produce calcite in the enormous quantities necessary—and the presence of zooxanthellae. This implies that reef building is energetically demanding. The zooxanthellae are thought to be one explanation for why reef-building corals are common in the beautifully clear, but nutritionally impoverished, waters of the tropical oceans: they have a virtually unlimited source of energy (light captured as glucose) supplied to them by their protozoan house guests.

The odd thing about this idea is that there is nothing in the process of depositing calcite that should require the coral to expend energy. Calcite is a weakly soluble salt of calcium and carbonate ions:

calcium ions in solution + carbonate ions in solution — calcite

"Weakly soluble" means that calcite has a strong tendency to form as a solid crystal (to go to the right side of the reaction) even if its soluble ions (on the left side of the reaction), are present in very small quantities: for calcium carbonate the required concentration is only about 95 X 10-6 molar. The weak solubility of calcite means that the process of its deposition in a reef should have the Second Law on its side: just put calcium ions and carbonate ions together and presto! you have calcite. Corals certainly have no problem obtaining the ingredients in sufficient quantities. In the oceans, calcium concentrations are roughly 100 times higher than they need be to support the formation of calcite. Neither is there any shortage of carbonate, which is abundant whenever there is metabolism. Wherever there is something living, there should always be plenty of carbonate.

So why should reef deposition be energetically costly? The answer lies in some peculiarities of the chemistry of carbonate when it is dissolved in water. What follows is the complete process of dissolving car bon dioxide (the product of glucose metabolism, remember) in water:

carbon dioxide + water « carbonic acid

« proton + bicarbonate « protons + carbonate

In this formula, the double-headed arrows signify that the reaction is reversible. This is a fairly complicated reaction, but its lesson for our problem is simple. Carbon dioxide does not dissolve in water like oxygen does—that is, as a solute that remains uninvolved, so to speak, with its solvent. Rather, carbon dioxide reacts with its solvent, water, to form a weak acid, carbonic acid (H2CO3). Carbonic acid can break apart into hydrogen ions (protons) and a hydrogenated form of carbonate, known as bicarbonate (HCO3-). To make carbonate in the form that calcium must pair with to make calcite, that hydrogen ion must be stripped off the bicarbonate.

Energy is required because the form that solubilized carbon dioxide most likes to be in is bicarbonate. If we were to use relative type size to express the relative abundance of the different forms carbonate tends to take, the reaction above would look something like this:

There is another problem with bicarbonate, this one for the zooxanthellae. For photosynthesis to occur, the zooxanthellae must have carbon dioxide. Not bicarbonate, not carbonate, not carbonic acid, but carbon dioxide. Even if the coral's metabolism produces prodigious quantities of CO2, the zooxanthellae will be starved if this CO2 is immediately locked up into the bicarbonate form.

So both the coral and the zooxanthellae have to work against the thermodynamically favored tendency of carbon dioxide, from one side, or carbonate, from the other, to form bicarbonate. Furthermore, they must work against this tendency in opposite directions. To deposit calcite, the coral must drive the reaction to the right, toward CO32-. To photosynthesize glucose, the zooxanthellae must drive the reaction to the left, toward CO2. (For a brief outline of chemical kinetics and the relevant principles behind the energetics of chemical reactions, see Box 2C).

Fortunately, there is a simple way to solve both problems, and it involves manipulating the concentration of hydrogen ions around the bicarbonate. Removing hydrogen ions from a solution of bicarbonate (making it more alkaline) forces the bicarbonate to get rid of its second hydrogen atom and form carbonate. Again, if we represent relative concentrations by type size, the reaction would look like this:

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