Thus, the burrow is a bit like the transistor-based structure described in Fig. 3.5. By directing a little met abolic energy into building and operating the burrow—namely, the cost of digging it and ventilating it— the worm activates a much larger flow of energy down an external potential energy gradient. This is not all the burrow does, though—it can also act as a rectifier (Fig. 3.5), a device that selectively impedes or allows a flux of matter one way but not the other.
Lugworms, when they construct their burrows, stabilize its walls, both by simple compaction and by secreting a coat of mucus that penetrates two or three centimeters into the sediments surrounding the burrow walls. The sticky mucus acts as a mortar, literally gluing grains of sediment together. Mucus has other peculiar properties, though, which make the burrow wall act as a metabolic rectifier. To understand why, you must first understand something about a chemical technique known as chromatography, one of a class of methods used by chemists to separate similar molecules from one another.
In chromatography, similar molecules are separated by differences in the interactions between the molecules in solution and a substrate that can bind them. We will use as an example the simplest type of chro-matography, paper chromatography, which draws a solution through a piece of absorbent paper in just the same way water is drawn up into a paper towel. Imagine that we wish to use paper chromatography to separate a solution of different amino acids from one another. The twenty-three amino acids differ from one another by the identity of a so-called R-group. In glycine, for example, the R-group is a hydrogen atom. The R-group of alanine, on the other hand, is a methyl group (CH3). The rest of the molecule is the same for all the other amino acids.
Remember, now, that paper is composed of numerous fibers of cellulose woven together into a tangled sheet, with spaces between the fibers. Imagine that we have a solution of amino acids being drawn through this array. The amino acid molecule, when it is in solution, is surrounded by a "jacket" of water molecules, forcibly held there by weak electrostatic forces between the water molecules and the amino acid mole cule. At the same time, there will be some attraction between the cellulose fibers and the amino acid molecules. The relative strengths of these interactions will depend upon the R-group. Some amino acids will make the water bind more avidly to them, some less so, and some will bind the cellulose fibers more avidly, others less so. When the solution moves past the fibers, there will be a competition between the water molecules and the cellulose fibers to see which can bind the amino acid molecule more strongly. If the attraction to cellulose is strong enough to overcome the forces holding the water molecules, the amino acid will be delayed slightly as the water carrying it flows past the fiber. If the attraction to cellulose is weaker, the amino acid will be less likely to tarry. Thus, molecules with a relatively weaker attraction for the cellulose will be carried ahead of those with a stronger attraction. Carried over many centimeters, the different amino acids will be separated from one another.
Cellulose can act as a medium for chromatography because it is a polysaccharide—that is, it is a polymer of glucose molecules. Glucose polymers are favorite tools for chromatography because of a curious distribution of charges that occurs in sugar molecules. Sugar molecules, while electrically neutral, nevertheless have some regions where the electrons spend more time than others. The charge distribution on a sugar molecule is therefore uneven: some regions, where the electrons are gathered, tend to be more negatively charged than regions where they are not. When a sugar is incorporated into a polysaccharide, and the polymer folds up into balls or sheets, and the polysaccharide presents an array of negative charges to the outside. The charged regions can then interact with charged molecules flowing by, slowing down some molecules and letting others past unimpeded.
The burrow lining secreted by a lugworm is, as I have said, mucus, which is mucopolysaccharide, a special type of protein-containing polysaccharide. Mucopolysaccharides also make good chromato-graphic media. The mere presence of a burrow lining impedes the flow of all materials across the burrow walls by 40 percent or so. Some molecules are impeded more than others, however. Negatively charged solutes, like bromide ion (Br-), are impeded more than positively charged ions, like ammonium (NH4+) that are similar in size and strength (but not sign) of the charge they carry. The burrow lining seems to allow positively charged solutes through more readily than negatively charged solutes.
In the environment around the burrow, the burrow lining's chromatographic properties manage the flow of oxidants and nutrients around the burrow. They can have far-reaching effects on the microbial communities surrounding the burrow. Consider, for example, the fate of ammonia, produced as a waste product whenever animals use protein as food.
Ammonia itself is a neutrally charged molecule, but in solution it reacts strongly with water to form ammonium hydroxide, which further dissociates into ammonium ions (NH4+) and hydroxyl (OH-):
Ammonium is highly toxic to all animals, because it reacts with one of the intermediates in the aerobic oxidation of glucose, diverting it (and its electrons) away from the production of ATP. Consequently, animals go to great lengths to get rid of ammonium, usually by literally flushing it away. Lugworms usually flush ammonia away by ventilating their burrows with water, but they do not ventilate their burrows all the time. During the unventilated periods, ammonium can accumulate around the worm.
Because the burrow lining is relatively permeable to positive ions, the accumulating ammonium can diffuse easily out of the burrow into the surrounding sediments (Fig. 6.9). Once outside the burrow, it serves as a nutrient for ammonia-oxidizing bacteria, which produce the negatively charged nitrite ion (NO2-) as a waste product. You will recall that the burrow lining impedes the flow of negatively charged ions across it, so the nitrite will tend to stay in the sediment rather than diffusing back into the burrow. Thus, the burrow
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