Justinian's Flea by William Rosen

Author:William Rosen [Rosen, William]
Language: eng
Format: epub
Tags: History
Published: 2011-11-06T13:05:09+00:00

Genes are so central to life as we know it that some evolutionary biologists, Richard Dawkins most prominently, are sometimes quoted as saying that organisms are nothing more than vehicles used by genes to assure their own propagation. But they're not everything. Reordering of DNA is necessary, but it is not sufficient. The other minimal requirement for governing a network of actions and reactions, whether chemical or human, is some border between self and everything else. Life needs walls.

Bacteria are defined by their walls. Literally: A bacterium whose membrane turns purple and stays that way when a crystal violet stain is applied is called gram-positive, while those that turn pink when a different solution is added are gram-negative. The term* is more than a naming convenience.

* It has nothing to do with the metric measure, but is named for Hans Christian Gram, the Danish physician who discovered the phenomenon in 1884.

One of the earliest evolutionary advances of bacteria was the creation of a thick cell wall of carbohydrate polymers-- a fancy name for sugars--connected via proteins, effectively forming one giant molecule called peptidoglycan. Much later, different species of bacteria wrapped the peptidoglycan in a combination of fat, protein, and carbohydrates. The combination not only resists the purple stain, but gives gram-negative bacteria a sturdier defense against attacks aimed at the sugar-protein wall of peptidoglycan.

The membrane is more than protection, more even than a necessary boundary marker between self and environment. The initial function of the bacterial membrane was probably nothing more than regulating the movement of energy into the bacterial cell. From that relatively simple beginning to the bacteria of today is a difference far greater than that between a lean-to and a cathedral. The bacteria of today build walls of remarkable complexity in order to sustain dozens of ongoing chemical reactions. Each of those reactions obliges the walls to allow some of the environment's molecules to enter, and some of the cell's to escape. To control that movement, the walls incorporate a dynamic system of aqueducts and pumps to move nutrients in and out of the bacterial cell; some of the pumps use a charged molecule to push or pull molecules with an opposite charge into the cell. Other systems use proteins to open channels in the cell wall that permit the entrance or exit of only one material. Still others assemble a train of proteins that pass an amino acid from one to the other, while other proteins hold open the membrane channel.

Even though the walls of the earliest bacteria were far simpler than those of today, the size of the bacteria that they contained has been virtually unchanged for more than three billion years. This is because bacterial size--about one to two micrometers in diameter, one three-hundredth the width of the period at the end of this sentence--is a function of a mathematical cube-square law. As noted above, because the chemical reactions basic to life require the movement of molecules from the outside of a membrane to the cellular material inside, a high surface-to-volume ratio facilitates them.

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